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                                                                   Exhibit 10.16



                  FIRST AMENDMENT TO REVOLVING CREDIT AGREEMENT

                           dated as of August 19, 1999

                                      among

                            PS BUSINESS PARKS, L.P.,


                           THE LENDERS LISTED HEREIN,


                                       and

                     WELLS FARGO BANK, NATIONAL ASSOCIATION,
                                    as Agent





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                                TABLE OF CONTENTS

                                                                           Page


ARTICLE I.  THE AMENDMENTS....................................................1

         SECTION 1.1.  Definitions............................................1

         SECTION 1.2.  Extension Fee..........................................3

         SECTION 1.3.  Extension..............................................3

         SECTION 1.4.  Financial Information..................................4

         SECTION 1.5.  Intentially Deleted....................................4

         SECTION 1.6.  Minimum Tangible Net Worth.............................4

         SECTION 1.7.  Fixed Charge Coverage..................................4

         SECTION 1.8.  Financial Statements and Other Reports.................4

         SECTION 1.9.  Environmental Matters..................................5


ARTICLE II.  CONDITIONS TO EFFECTIVENESS OF THIS AMENDMENT....................6


ARTICLE III.  REPRESENTATIONS OF BORROWER.....................................7


ARTICLE IV.  MISCELLANEOUS....................................................7

         SECTION 4.1.  Capitalized Terms......................................7

         SECTION 4.2.  Ratification...........................................7

         SECTION 4.3.  Counterparts...........................................7

         SECTION 4.4.  Governing Law..........................................7

                                       i


                  FIRST AMENDMENT TO REVOLVING CREDIT AGREEMENT

          THIS FIRST AMENDMENT TO REVOLVING CREDIT AGREEMENT (this  "Amendment")
dated as of August 19, 1999 among PS BUSINESS PARKS,  L.P., a California limited
partnership (the  "Borrower"),  the lenders listed on the signature pages hereof
("Lenders"),   and  WELLS  FARGO  BANK,  NATIONAL  ASSOCIATION,   as  agent  and
representative for the Lenders (in such capacity, the "Agent").

          WHEREAS,  Borrower,  the Agent and the lenders listed on the signature
pages thereof entered into that certain  Revolving Credit  Agreement  ("Original
Agreement") dated as of August 6, 1998;

          WHEREAS,  Borrower,  the  Lenders  and the Agent  wish to  extend  the
Maturity  Date to  August  6,  2002 and make  certain  other  amendments  to the
Original Agreement. The Original Agreement, as modified by this Amendment may be
referred to herein as the "Credit Agreement";

          NOW,  THEREFORE,  in  consideration  of the premises and of the mutual
covenants and agreements  hereinafter set forth,  the Borrower,  the Lenders and
the Agent agree as follows:

                                   ARTICLE I.

                                 THE AMENDMENTS

          SECTION 1.1.  Definitions.  The following  terms shall be added to, or
shall be substituted in lieu of the  corresponding  terms in, Section 1.1 of the
Original Agreement:

          "Applicable  Margin" means,  with respect to each Loan, the respective
percentages  per annum  determined,  at any time,  based on the range into which
Borrower's  Credit  Rating then falls,  in  accordance  with the table set forth
below. Any change in Borrower's  Credit Rating causing it to move to a different
range on the table shall effect an  immediate  change in the  Applicable  Margin
(including  existing  Loans).  Promptly  after  learning  of  a  change  in  the
Borrower's Credit Rating,  Agent shall give notice of such change to the Lenders
and include in such notice the new  Applicable  Margin and the effective date of
such change.  In the event that more than one (1)  different  Credit  Rating has
been assigned, the lower of the Credit Ratings will prevail.

                                       1


                                     GRID A:



                                                            Applicable
                                                            Margin for               Applicable
                        Range of                            Base Rate                Margin for
                        Borrower's                          Loans                    LIBOR Loans
                        Credit Rating                       (% per annum)            (% per annum)
                        -------------                       -------------            -------------
                                                                            

Level I                 A-/A3
                        or better                                   0.0                    0.75

Level II                BBB+/Baa1                                   0.0                    0.80

Level III               BBB/Baa2                                    0.0                    0.95

Level IV                BBB-/Baa3                                   0.0                    1.00

Level V                 Unrated or Below Investment Grade           0.0              See Grid B


                                     GRID B:


                                                            Applicable
                                                            Margin for               Applicable
                                                            Base Rate                Margin for
                                                            Loans                    LIBOR Loans
                        Leverage                            (% per annum)            (% per annum)
                        -------------                       -------------            -------------
                                                                            

Level I                 Less than or Equal to 25%                   0.0                    1.05

Level II                > 25% Less than or Equal to 35%             0.0                    1.10

Level III               > 35% Less than or Equal to 45%             0.0                    1.20

Level IV                > 45%                                       0.0                    1.35


          "Equity  Offering  Net  Proceeds"  means,  cumulatively,  the Net cash
proceeds  received  and the value of assets  acquired  (net of Debt  incurred or
assumed in  connection  therewith)  through the issuance of Capital Stock of any
Borrower Party after the Amendment Date,  excluding any amounts  attributable to
mandatorily  redeemable  preferred stock (other than preferred stock  redeemable
solely  with  common  stock).  "Net"  means  net  of  underwriters'   discounts,
commission  and other  reasonable  out-of-pocket  expenses  actually paid to any
Person (other than any Borrower Party or any Affiliate thereof).

          "Fee Letter"  means that certain  letter dated August 19, 1999 between
the Borrower and the Agent.

          "Fixed   Charges"  means,   for  any  Fiscal   Quarter,   and  without
duplication,  Interest Expense for such Fiscal Quarter, plus scheduled principal
amortization  payments  (other than  balloon  payments)  on Debt of the Borrower
Parties and the  Consolidated  Entities  during such  Fiscal  Quarter,  plus the
Capital  Expenditure  Reserve,  plus all dividends and other  distributions paid
during  such  Fiscal  Quarter  to  holders  of  preferred   stock  or  preferred
partnership units of the Borrower Parties and the Consolidated Entities.

                                       2


          "Liquidated Cost" shall have the meaning set forth in Section 5.12.

          "Maturity Date" means at any time, the  then-applicable  maturity date
specified hereunder. The initial Maturity Date shall be August 6, 2002, although
such date may be extended by the Lenders as provided in Section 2.5.2 hereof.

          "Revolving Loan Note" means a Note made by the Borrower payable to the
order  of a  particular  Lender,  in  the  amount  of  such  Lender's  Revolving
Commitment,  which note is  substantially in the form of Exhibit A-1, as amended
(including any amendments and restatements thereof) from time to time.

          SECTION 1.2. Extension Fee. Section 2.4.2 of the Original Agreement is
hereby deleted in its entirety.

          SECTION 1.3.  Extension.  Section  2.5.2 of the Original  Agreement is
hereby  deleted in its entirety and the following  shall be  substituted in lieu
thereof:

          2.5.2.  Extension.  Borrowers  may request  extensions of the Maturity
Date by making such  request to Agent  ("Extension  Notice") in writing at least
ninety (90) days prior to each  anniversary of the Closing Date (commencing with
the  anniversary  falling on August 6, 2000).  The Agent and the Lenders have no
obligation  to extend  the  Maturity  Date and the  Maturity  Date  shall not be
extended  unless (i) the Borrower is in full  compliance  with all of the terms,
conditions  and  covenants  of this  Agreement at the time of request and on the
applicable  anniversary  Date, (ii) all of the Lenders and the Agent have agreed
to do so in  writing,  (iii)  Borrower  shall,  on or  prior  to the  applicable
anniversary,  have executed and delivered to the Agent an extension agreement in
the  form  provided  by  Agent,  and  (iv)  Borrower  shall,  on or prior to the
applicable  anniversary,  provided all Lenders  shall have approved the request,
have  remitted to the Agent any  extension  fee,  and have  satisfied  any other
conditions to extension, agreed to between Borrower and the Agent. If Borrower's
request for extension is approved and the other  foregoing  conditions  are met,
then (i) the  extension  of the  Maturity  Date shall be for a period of one (1)
year  and  (ii)  such  extension   shall  be  effective  as  of  the  applicable
anniversary.  The Agent and the Lenders shall have a period of  forty-five  (45)
days from  receipt  of  written  notice of  Borrowers'  intention  to extend the
Maturity Date to approve such extension,  in their sole and absolute discretion.
If Borrower has not received written notice of the Lenders'  intention to extend
the Maturity  Date within such  forty-five  (45) day period,  then the extension
request shall be deemed to be not approved. If an extension is granted, Borrower
may request  subsequent one (1) year extensions subject to the same criteria and
procedures  established in this Section 2.5.2. As an example, in order to extend
the initial Maturity Date,  Borrower must notify Agent at least ninety (90) days
prior to August 6, 2000.  If approved,  the Maturity Date would then be extended
from  August 6, 2002 to August 6,  2003.  In the event that  Borrower's  initial
request for extension is not granted,  any  subsequent  request for extension is
not granted,  or Borrower does not request an extension pursuant to this Section
2.5.2, then,  commencing on the Maturity Date,  Borrower shall no longer be able
to obtain Loans  hereunder  and all  outstanding  Loans shall become all due and
payable.

                                       3


          SECTION  1.4.  Financial  Information.  Section  4.5.1 of the Original
Agreement  is hereby  amended by  deleting  the dates  "December  31,  1996" and
"December  31, 1997" and  substituting  in lieu thereof the dates  "December 31,
1997" and  "December  31,  1998"  respectively.  Section  4.5.2 of the  Original
Agreement is hereby amended by deleting the words  "September 30, 1997 and March
31, 1998" and substituting in lieu thereof the words "June 30, 1999".

          SECTION 1.5. [Intentially Deleted].

          SECTION 1.6. Minimum Tangible Net Worth. Section 6.4.3 of the Original
Agreement  is  hereby  deleted  in its  entirety  and  the  following  shall  be
substituted in lieu thereof:

          6.4.3. Minimum Tangible Net Worth.  Tangible Net Worth of Borrower and
Guarantor shall not be less than, at any time: (i) $675,000,000 plus (ii) ninety
percent (90%) of Equity Offering Net Proceeds.

          SECTION 1.7.  Fixed  Charge  Coverage.  Section  6.4.6 of the Original
Agreement  is  hereby  deleted  in its  entirety  and  the  following  shall  be
substituted in lieu thereof:

          6.4.6.  Fixed  Charge  Coverage.  At any time,  the ratio of EBITDA to
Fixed Charges for the most recently  completed  Fiscal Quarter shall not be less
than 1.75:1.0.

          SECTION 1.8.  Financial  Statements and Other Reports.  Sections 5.1.2
through 5.1.5 of the Original Agreement are hereby deleted in their entirety and
the following shall be substituted in lieu thereof:

          5.1.2.  As soon as practicable and in any event within fifty (50) days
after the end of each of the first three (3) Fiscal  Quarters during each Fiscal
Year a consolidated  balance sheet of the Borrower Parties as of the end of such
quarter and the related consolidated statements of income,  stockholders' equity
and cash flow for such  quarter  and the portion of the Fiscal Year ended at the
end of such  quarter,  setting  forth  in  each  case in  comparative  form  the
consolidated figures for the corresponding periods of the prior Fiscal Year, all
in reasonable detail and certified by the Guarantor's chief financial officer as
fairly presenting the consolidated  financial  condition of the Borrower Parties
as of the dates  indicated and the  consolidated  results of operations and cash
flows for the periods indicated, subject to normal year-end adjustments and made
in accordance with GAAP.

          5.1.3.  Within  ninety-five  (95) days  after  the end of each  Fiscal
Quarter  ending  December  31 and  within  fifty (50) days after the end of each
other Fiscal  Quarter,  a certificate  of the senior  vice-president,  corporate
finance,  chief  financial  officer,  controller  or treasurer of the  Guarantor
substantially  in the form of Exhibit F (a "Compliance  Certificate"),  (a) duly

                                       4

completed setting forth the calculations required to establish  Availability and
compliance  with Section 6.4 on the date of such  financial  statements  and (b)
stating that, to the best  knowledge of such officer,  after making such inquiry
and  other   investigation   as  such  officer   deems   reasonable   under  the
circumstances, no Default exists or, if a Default does exist, the nature thereof
and the action that the Borrower proposes to take with respect thereto;

          5.1.4.  Within  ninety-five  (95) days  after  the end of each  Fiscal
Quarter  ending  December  31 and  within  fifty (50) days after the end of each
other Fiscal Quarter, a report showing Available Financing as of the end of such
Fiscal Quarter.

          5.1.5.   An   Unencumbered   Pool  report  which   includes  for  each
Unencumbered  Asset,  the Property NOI for such Fiscal  Quarter with  reasonable
detail as to all Property Expenses,  Capital Expenditures  incurred, and average
Occupancy  Rate during the Fiscal  Quarter.  This portion of the report shall be
submitted to the Agent within ninety-five (95) days after the end of each Fiscal
Quarter  ending  December  31 and  within  fifty (50) days after the end of each
other Fiscal Quarter.

          SECTION 1.9.  Environmental  Matters.  Section  5.12.1 of the Original
Agreement  is  hereby  deleted  in its  entirety  and  the  following  shall  be
substituted in lieu thereof:

          5.12.1.  Promptly upon discovery of any violation or alleged violation
of Environmental  Requirements with respect to any Real Property of any Borrower
Party,  the  Borrower  shall  attempt  in good faith as soon as  practicable  to
determine the cost to remediate such violation of Environmental Requirements and
the  Borrower  shall  thereupon  notify the Agent in  writing of the  Borrower's
reasonable,  good faith  estimate of the cost to  remediate  such  violation  or
alleged  violation.  Such  good  faith  estimate  of  the  cost  of  remediation
(exclusive of costs and expenses of investigation), as revised from time to time
pursuant hereto,  shall be deemed to be the "Liquidated  Cost" of such violation
or  alleged  violation  of  Environmental   Requirements.   From  time  to  time
thereafter,  not less than  ninety-five  (95) days after the end of each  Fiscal
Quarter  ending  December  31 and not less than fifty (50) days after the end of
each other Fiscal  Quarter,  the Borrower shall review and update all Liquidated
Costs  and  shall  deliver a written  report  to the  Agent  setting  forth,  in
reasonable  detail,  each  Liquidated  Cost in  excess  of One  Million  Dollars
($1,000,000),  the basis for the  determination  of the Liquidated Cost, and the
Borrower's  plans  with  respect  to such  violation  or  alleged  violation  of
Environmental Requirements.

                                  ARTICLE II.

                  CONDITIONS TO EFFECTIVENESS OF THIS AMENDMENT

          The  closing  hereunder  shall  occur  on the  date  when  each of the
following  conditions is satisfied (or waived by the Agent and the Lenders) (the
"Amendment Date"), each document to be dated the Amendment Date unless otherwise
indicated:

                                       5


          (a) the Borrower  shall have  executed and delivered to the Agent duly
executed  original  Notes for the account of each Lender dated as of the Closing
Date complying with the provisions of Section 2.3 of the Credit Agreement;

          (b) the  Borrower,  the  Agent  and  each of the  Lenders  shall  have
executed and delivered to the Borrower and the Agent a duly executed original of
this Amendment;

          (c)  Guarantor  shall have  executed and delivered to the Agent a duly
executed consent to this Amendment reaffirming Guarantor's obligations under the
Guaranty;

          (d) the  Agent  shall  have  received  all  documents  the  Agent  may
reasonably request relating to the existence of the Borrower and Guarantor,  the
authority for and the validity of this  Amendment and the other Loan  Documents,
and any other matters relevant hereto, all in form and substance satisfactory to
the Agent. Such documentation shall include,  without limitation,  the agreement
of limited  partnership of the Borrower,  as well as the  certificate of limited
partnership of the Borrower,  both as amended,  modified or  supplemented to the
Amendment Date,  certified to be true,  correct and complete by a senior officer
of the Borrower as of a date not more than ten (10) days prior to the  Amendment
Date,  as well as the  articles of  incorporation  and bylaws of  Guarantor,  as
amended,  modified or supplemented to the Amendment Date,  certified to be true,
correct and complete by a senior officer of Guarantor as of a date not more than
ten (10) days prior to the Amendment Date;

          (e) the Borrower and Guarantor  shall have taken all actions  required
to authorize  the  execution  and delivery of this  Amendment and the other Loan
Documents and the performance thereof by the Borrower and Guarantor, as the case
may be;

          (f) the Agent  shall  have  received,  for its and any other  Lender's
account and the account of Gibson, Dunn & Crutcher LLP, all fees due and payable
pursuant to the Lender Fee Letter on or before the Amendment Date;

          (g) the Borrower shall have executed and delivered to the Agent a duly
executed original of the Fee Letter;

          (h) no Default or Event of Default shall have occurred; and

          (i) each of the Notes  executed  by Borrower  in  connection  with the
Original  Agreement  shall have been  surrendered by the relevant  Lender to the
Agent for  cancellation  and  return  to the  Borrower  simultaneously  with the
Closing  (it  being  acknowledged  and  agreed  by the  Lenders  that the  Notes
originally  executed by Borrower as of the Closing Date in  connection  with the
Original Agreement (which are being replaced as of the Amendment Date by amended
and  restated  notes) shall be deemed  canceled,  paid in full and of no further
force and effect as of the Amendment Date.

                                       6


                                  ARTICLE III.

                           REPRESENTATIONS OF BORROWER

          The Borrower  hereby  represents and warrants to the Agent and each of
the Lenders the following:

          (a)  All  of  the  representations  and  warranties  contained  in the
Original Agreement are true and correct on and as of the date hereof and will be
true  and  correct  after  giving  effect  to  this  Amendment;   the  foregoing
representation  and  warranty  is not  intended to modify  Section  7.1.4 of the
Credit Agreement.

          (b) No event which  constitutes a Default or an Event of Default under
the Original  Agreement,  as amended hereby, has occurred and is continuing,  or
would result from the execution and delivery of this Amendment.

          (c) The  Borrower  has the power and  authority to execute and deliver
this Amendment and to perform its obligations under the Original  Agreement,  as
amended  hereby,  and  under  the  Notes;  and all such  action  has  been  duly
authorized  by all  necessary  proceeding  on its  part.  Each  of the  Original
Agreement,  this Amendment and the Notes has been duly and validly  executed and
delivered  by the  Borrower  and  constitutes  the  valid  and  legally  binding
obligation of the Borrower  enforceable in accordance with its terms,  except as
limited by  moratorium,  bankruptcy,  reorganization,  insolvency  or other laws
affecting  creditor's rights generally or by the exercise of judicial discretion
in accordance with general principles of equity.

                                   ARTICLE IV.

                                  MISCELLANEOUS

          SECTION 4.1 Capitalized  Terms The capitalized terms used herein which
are defined in the Original  Agreement  and not otherwise  defined  herein shall
have the meanings specified therein.

          SECTION 4.2 Ratification The Original Agreement, as hereby amended, is
in all respects ratified and confirmed,  and all other rights and powers created
thereby or thereunder shall be and remain in full force and effect.

          SECTION 4.3  Counterparts  This  Amendment  may be executed in several
counterparts,  and each  counterpart,  when so  executed  and  delivered,  shall
constitute  an original  instrument,  and all such separate  counterparts  shall
constitute one and the same instrument.

          SECTION 4.4 Governing Law THIS  AMENDMENT AND THE OTHER LOAN DOCUMENTS
AND THE RIGHTS AND OBLIGATIONS OF THE PARTIES  HEREUNDER AND THEREUNDER SHALL BE
CONSTRUED  IN  ACCORDANCE  WITH  AND BE  GOVERNED  BY THE  LAWS OF THE  STATE OF
CALIFORNIA  EXCEPT TO THE EXTENT PREEMPTED BY FEDERAL LAW (WITHOUT GIVING EFFECT
TO THE PRINCIPLES THEREOF RELATING TO CONFLICTS OF LAW).

                                      7


          IN WITNESS  WHEREOF,  the parties hereto have caused this Amendment to
be duly executed by their respective  authorized officers as of the day and year
first above written.

                                        Borrower:

                                        PS BUSINESS PARKS, L.P.,
                                        a California limited partnership

                                        By: PS BUSINESS PARKS, INC.,
                                        a California corporation,
                                        General Partner


                                        By: /s/ Ronald L. Havner
                                            ----------------------
                                            Name: Ronald L. Havner
                                            Title: President

                                        Address: PS BUSINESS PARKS, L.P.
                                        701 Western Avenue
                                        Glendale, California 91201
                                        Attn: Chief Financial Officer
                                        Telephone: (818) 244-8080
                                        Telecopier:(818) 244-9267


                                        Agent:

                                        Wells Fargo Bank, National Association


                                        By: /s/ Sharon Fisher
                                            ----------------------
                                            Name: Sharon Fisher
                                            Title: Vice President

                                        Address: Wells  Fargo  Bank,
                                                 National Association
                                        2030 Main Street, 8th Floor
                                        Irvine, California 92614
                                        Attention: Office Manager
                                        Telephone: (949) 251-4300
                                        Telecopier: (949) 851-9728


                                        Lender:

                                        Wells Fargo Bank, National Association


                                        By: /s/ Sharon Fisher
                                            ----------------------
                                            Name: Sharon Fisher
                                            Title: Vice President

                                        Address: Wells Fargo Bank,
                                                 National Association
                                        2030 Main Street, 8th Floor
                                        Irvine, California 92614
                                        Attention: Office Manager
                                        Telephone: (949) 251-4300
                                        Telecopier: (949) 851-9728

                                        LIBOR LENDING OFFICE:
                                        Wells Fargo Bank, National Association
                                        2120 East Park Place, Suite 100
                                        El Segundo, California 90245
                                        Attention: Anne Colvin
                                        Telephone: (310) 335-9458
                                        Telecopier: (310) 615-1014




                              CONSENT OF GUARANTOR

The   undersigned,   PS  BUSINESS   PARKS,   INC.,  a   California   corporation
("Guarantor"), (i) hereby consents to the foregoing First Amendment to Revolving
Credit  Agreement dated as of August 19, 1999 (the "First  Amendment")  among PS
BUSINESS PARKS, L.P., a California limited partnership ("Borrower"), the lenders
listed therein (the "Lenders") and WELLS FARGO BANK,  NATIONAL  ASSOCIATION,  as
Agent (in such capacity, the "Agent"), and (ii) hereby reaffirms its obligations
under that certain General  Continuing  Repayment Guaranty dated as of August 6,
1998 made by Guarantor in favor of the Lenders and the Agent  pursuant to which,
among  other  things,  Guarantor  guarantees  the  payment  and  performance  of
Borrower's  obligations  under the Revolving Credit Agreement dated as of August
6, 1998 among  Borrower,  the  Lenders  and the  Agent,  as amended by the First
Amendment.

                                        PS BUSINESS PARKS, INC.,
                                        a California corporation


                                        By: /s/ Ronald L. Havner
                                            ----------------------
                                            Name: Ronald L. Havner
                                            Title: President