EXHIBIT 2.3


                AMENDMENT NO. 2 TO AGREEMENT AND PLAN OF MERGER
                                  BY AND AMONG
                           AMERICAN TOWER CORPORATION,
                             ATI MERGER CORPORATION,
                                AND UNISITE, INC.

     This Amendment No. 2 ("Second Amendment"), effective as of December 31,
1999, is to the Agreement and Plan of Merger by and among American Tower
Corporation ("ATC"), ATI Merger Corporation ("ATI"), and UniSite, Inc.
("UniSite"), dated June 28, 1999, as amended (the "Merger Agreement").

                                   BACKGROUND

     The parties desire to provide for the Merger to close as of January 10,
2000.  Because UniSite may have less than 600 Completed Towers (as defined in
the Merger Agreement) as of January 10, 2000, the parties desire to adjust the
Merger Consideration (as defined in the Merger Agreement) to reflect the cost to
complete a number of towers necessary for UniSite to have 600 Completed Towers
(as defined in the Merger Agreement) at the closing of the merger, which shall
be based on the least expensive towers that UniSite could complete by January
31, 2000.  The parties also desire to provide a mechanism by which the parties
can agree as to the amount of the Working Capital component of Merger
Consideration instead of submitting disputes regarding working capital to an
independent accounting firm.

     NOW, THEREFORE, in consideration of the mutual covenants contained in this
Second Amendment and other good and valuable consideration, the parties hereby
agree as follows:

1.  Capitalized Terms.  Capitalized terms used but not defined in this Second
Amendment shall have the meaning given to them in the Merger Agreement.

2.  Amendment of the Merger Agreement.

     (a)  The Merger Agreement is amended as follows:

          (i)  Section 2.2 shall be amended in its entirety to read as follows:

     "2.2  Closing.  Unless this Agreement shall have been terminated pursuant
     to Section 8.1 and subject to the satisfaction or, to the extent permitted
     by Applicable Law, waiver of the conditions set forth in Article 7, the
     closing of the Merger (the "Closing") will take


     place, at 12:00 p.m., on the Closing Date, at the offices of Sullivan &
     Worcester LLP, One Post Office Square, Boston, Massachusetts 02109, on
     January 10, 2000, unless another date, time, or place is agreed to in
     writing by the parties. The date on which the Closing occurs is herein
     referred to as the "Closing Date."

               (ii) The definition of "Merger Consideration" contained in
                    Section 3.1 shall be amended in its entirety to read as
                    follows:

     "The term "Merger Consideration" shall mean an amount equal to the sum of:

          (i) $205 million less the Merger Consideration Adjustment;

          (ii) increased or decreased, as the case may be, by an amount equal to
          the positive or negative Working Capital of UniSite as of the
          Effective Time;

          (iii) decreased by an amount equal to the excess of (A) the principal
          amount of Indebtedness for Money Borrowed of UniSite and its
          Subsidiaries as of the Effective Time over (B) the principal amount of
          Indebtedness for Money Borrowed of UniSite and its Subsidiaries as of
          the Effective Time that was borrowed subsequent to the date of this
          Agreement and used to fund construction of towers (other than
          Completed Towers and Deemed Completed Towers) subsequent to the date
          of this Agreement; and

          (iv) all adjustments, if any, pursuant to the provisions of Sections
          7.2(d),(f),(j) and (l).


                    (iii) Section 3.5 shall be amended in its entirety to read
          as follows:

     "3.5  Determination of Merger Consideration.
           -------------------------------------

     UniSite shall submit in writing to ATC by 5:00 p.m. Eastern Standard Time
on January 5, 2000, a notice containing the Merger Consideration Adjustment and
UniSite's estimate of the amount of Working Capital as of the Closing Date
("UniSite's Working Capital Estimate"). ATC may examine all records used to
prepare UniSite's Working Capital Estimate. At the Effective Time, ATC shall pay
to the Merger Trust in immediately available funds by wire transfer to a bank
account specified by the trustees of the Merger Trust the amount of the Merger
Consideration determined in


accordance with Section 3.1(without regard to Working Capital) less $ 2 million,
increased or decreased, as the case may be, by the sum of UniSite's Working
Capital Estimate. Notwithstanding the foregoing, ATC may increase or decrease,
as the case may be, UniSite's Working Capital Estimate by an amount that ATC
specifies in a notice provided to UniSite by 6:00 p.m. Eastern Standard Time on
January 6, 2000 equal to ATC's reasonable, good faith objections to UniSite's
Working Capital Estimate. The consideration that ATC pays to the Merger Trust at
the Effective Time is referred to as the "Preliminary Merger Consideration" and
is subject to post-Effective Time adjustments, if any, as provided in this
Section.

(i)  If ATC specifies changes to UniSite's Working Capital Estimate in the
     prescribed manner, then within fifteen(15) days after the Closing Date, the
     UniSite Stockholder Representatives shall cause to be prepared and
     delivered to ATC a statement of Working Capital as of the Closing Date (the
     "Preliminary Working Capital Statement").  ATC shall provide to the UniSite
     Stockholder Representatives and their designees access to all relevant
     books, records, and personnel of UniSite to facilitate the preparation of
     the Preliminary Working Capital Statement.  If within ten (10) days
     following delivery of the Preliminary Working Capital Statement to ATC, ATC
     has not given the UniSite Stockholder Representatives notice of its
     objection to the Preliminary Working Capital Statement (such notice must
     contain a reasonable description of the basis of ATC's objection), then the
     Working Capital reflected on the Preliminary Working Capital Statement
     shall be deemed binding and conclusive on all parties to this Agreement and
     the UniSite securityholders, shall not be subject to dispute or review, and
     shall become the "Final Working Capital."  If ATC gives notice of objection
     to the Preliminary Working Capital Statement within the ten-day period,
     then the issues in dispute shall be submitted to a nationally-known
     independent public accounting firm, to be selected jointly by the UniSite
     Stockholder Representatives and ATC, or if they cannot agree by KPMG Peat
     Marwick LLP (the "Accountants") within ten (10) days following the UniSite
     Stockholder Representatives' receipt of notice of ATC's objection to the
     Preliminary Working Capital Statement, for resolution. The Accountants
     shall determine the "Final Working Capital" within thirty (30) days after
     the dispute is submitted to them.  If issues in dispute are submitted to
     the Accountants for resolution pursuant to this subsection or subsection
     (iii) below, then (A) each party will furnish to the Accountants such work
     papers and other documents and information relating to the disputed issues
     as the Accountants may request and are available to that party


     or its Subsidiaries (or its independent public accountants), and will be
     afforded the opportunity to present to the Accountants any material
     relating to the determination and to discuss the determination with the
     Accountants; (B) the determination by the Accountants of Final Working
     Capital as set forth in a notice delivered to both parties by the
     Accountants, will be binding and conclusive on the parties and the UniSite
     securityholders and shall not be subject to dispute or review; and (C) ATC
     will bear the fees and expenses of the Accountants for such determination.

     (ii) On the third business day following the determination of the Final
     Working Capital, if the amount of the Final Working Capital is greater than
     the Working Capital portion of Preliminary Merger Consideration, then ATC
     will pay the difference between such amounts to the Merger Trust in
     immediately available funds by wire transfer to a bank account specified by
     the trustees of the Merger Trust and (B) if the amount of the Final Working
     Capital is less than the amount of the Working Capital contained in the
     Preliminary Merger Consideration, then the Merger Trust shall pay to ATC
     the difference between such amounts in immediately available funds by wire
     transfer to a bank account specified by ATC.

     (iii)  If within fifteen (15) days following the Closing Date, ATC has not
     given the UniSite Stockholder Representatives written notice ("ATC's Cost
     to Complete Notice") of its objection to the Merger Consideration
     Adjustment (such notice to contain a reasonable description of the basis of
     ATC's objection, including adjustments, if any, described in Sections
     7.2(d), (f), (j), and (l)), then on the third business day following such
     fifteen (15)-day period, ATC will pay to the Merger Trust in immediately
     available funds by wire transfer to a bank account specified by the
     trustees of the Merger Trust the sum of $2 million, together with interest
     of five percent (5%) per annum from the Closing Date to such payment.  If
     within fifteen (15) days following the Closing Date, ATC gives to the
     UniSite Stockholder Representatives ATC's Cost to Complete Notice and the
     UniSite Stockholder Representatives do not object to any amounts contained
     in ATC's Cost to Complete Notice within five (5) days following the UniSite
     Stockholder Representatives' receipt thereof, then on the third (3rd)
     business day after such five (5)-day period, ATC will pay to the Merger
     Trust in immediately available funds by wire transfer to a bank account
     specified by the trustees of the Merger Trust the excess, if any, of $2
     million over the amount that ATC disputes in ATC's Cost to Complete Notice,
     together with interest of five percent (5%) per annum from the Closing Date
     to such payment.  If the UniSite Stockholder Representatives dispute in
     writing any of the


     amounts contained in ATC's Cost to Complete Notice within five (5) days
     following the UniSite Stockholder Representatives' receipt thereof, then
     the issues in dispute shall be submitted to the Accountants, within ten
     (10) days thereafter, for resolution. The Accountants shall determine the
     "Final Merger Consideration Adjustment" within thirty (30) days after the
     dispute is submitted to them and such determination will be binding and
     conclusive on the parties and the UniSite securityholders and shall not be
     subject to dispute or review. On the third business day following the
     determination of the Final Merger Consideration Adjustment, ATC will pay to
     the Merger Trust in immediately available funds by wire transfer to a bank
     account specified by the trustees of the Merger Trust $2 million; minus the
     excess, if any, of the Final Merger Consideration Adjustment over the
     Merger Consideration Adjustment; or plus the excess, if any, of the Merger
     Consideration Adjustment over the Final Merger Consideration Adjustment,
     together with interest of five percent (5%) per annum from the Closing Date
     to such payment.

          (iv) Appendix A to the Merger Agreement shall be amended by adding the
     following definitions:

          "ATC's Cost to Complete Notice" shall have the meaning given to it in
          Section 3.5.

          "Accountants" shall have the meaning given to it in Section 3.5.

          "Deemed Completed Tower" shall mean any towers that are the subject of
          the Merger Consideration Adjustment.

          "Final Merger Consideration Adjustment" shall have the meaning given
          to it in Section 3.5.

          "Final Working Capital" shall have the meaning given to it in Section
          3.5.

          "Merger Consideration Adjustment" shall mean an estimate, prepared by
          UniSite and not reasonably objected to by ATC; (it being understood,
          however, that ATC may only object in the manner prescribed in Section
          3.5 (iii)), of (a) the cost to complete construction of 600 towers
          less the number of UniSite's Completed Towers as of December 28, 1999,
          which, in the case of towers not yet erected shall be based on the 22
          least expensive towers that UniSite represents and warrants are
          scheduled for completion, in accordance with past


          practices and experience, by January 31, 2000, as identified by
          UniSite, and (b) the aggregate amount of adjustments, if any, referred
          to in Sections 7.2(d), (f), (j), and (l). Such estimate shall
          identify, as of December 28, 1999, the number of towers that were
          Completed Towers, the number of towers that were erected, but that
          require additional work to meet the "Completed Tower" definition, and
          the number of towers scheduled for completion by January 31, 2000.

          "Preliminary Working Capital Statement" shall have the meaning given
          to it in Section 3.5.

          "UniSite's Working Capital Estimate" shall have the meaning given to
          it in Section 3.5.


               (v) The definition of "Working Capital" contained in Appendix A
to the Merger Agreement is amended to read in its entirety as follows:

          "Working Capital" shall mean, with respect to UniSite, the amount by
          which the current assets of UniSite and its Subsidiaries exceed (or
          are less than) the current liabilities (other than the principal
          amount of Indebtedness for Money Borrowed and costs pertaining to
          completion of the Deemed Completed Towers incurred after December 28,
          1999) of UniSite and its Subsidiaries, as determined in accordance
          with GAAP, consistently applied with the UniSite Financial Statements,
          except as hereinafter specifically set forth; provided, however, that
          notwithstanding the foregoing:

          (a)  current assets shall be increased by an amount equal to the sum
               of:

               (i)  Tower Construction Costs actually paid by UniSite with
                    respect to Towers Under Construction (which shall not
                    include Deemed Completed Towers);

               (ii) funds actually paid to (x) CSFB for acting as financial
                    adviser to UniSite with respect to the subject matter of
                    this Agreement, (y) Fleet, for acting as financial adviser
                    to UniSite in connection with its sale of high-yield debt
                    securities in an amount not in excess of


                    $500,000, and to reimburse Fleet's reasonable costs and
                    expenses in connection therewith;

               (iii) funds actually paid with respect to all severance, accrued
                    vacation or other benefits payable to terminated UniSite
                    Employees as a result of the Merger (other than those paid
                    pursuant to the plan referred to in Section 6.14) to the
                    extent they do not, together with amounts accrued under
                    clause (b)(i)(z) following, exceed $2,000,000;

               (iv) funds actually paid by UniSite subsequent to the date of
                    this Agreement and prior to the Effective Time for
                    Environmental Reports and Structural Reports on Completed
                    Towers; and


     (b)  There shall be:
          (i)  excluded from current liabilities any accounts payable or accrued
               expenses incurred:

          (v)  by UniSite subsequent to the date of this Agreement and prior to
               the Effective Time for Environmental Reports and Structural
               Reports on Completed Towers;

          (w)  in respect of Tower Construction Costs with respect to Towers
               Under Construction;

          (x)  subsequent to the date of this Agreement in respect of
               acquisitions of towers (so long as such acquisitions were
               approved by ATC in accordance with the provisions of Section
               6.6);

          (y)  in respect of (I) CSFB for acting as financial adviser as
               described above, and (II) Fleet, for acting as financial adviser
               as described above in an amount not in excess of $500,000 (less
               any amount theretofore actually paid and reflected in clause
               (a)(ii)(y) immediately preceding), together with, without
               duplication, reimbursement of reasonable costs and expenses;

          (z)  in respect of all severance, accrued vacation or other benefits
               payable to terminated UniSite Employees (other than those paid
               pursuant to the


               plan referred to in Section 6.14) to the extent they do not,
               together with amounts paid under clause (a) (iii) above, exceed
               $2,000,000; and



               (ii) included in current liabilities an accrual with respect to
          the full amount of all costs and expenses required to be borne by
          UniSite pursuant to the provisions of this Agreement, including
          without limitation (w) those referred to in Section 10.2 (except
          amounts paid under clause (a)(iv) or excluded under clause (b)(i)(v)
          above), (x) all severance, accrued vacation or other benefits paid or
          payable (other than with respect to the plan referred to in Section
          6.14) to all terminated UniSite Employees in excess of $2,000,000,
          whether or not required by GAAP (it being understood that reserves
          will be established for any such obligation not accounted for as a
          liability), (y) all benefits payable pursuant to the plan referred to
          in Section 6.14 (it being understood that reserves will be established
          for any such obligation not accounted for as a liability); and (z) all
          amounts payable to holders of UniSite Stock Options referred to in
          Section 3.4; provided, however, that it is understood, and ATC hereby
          agrees, that the Surviving Corporation shall be responsible for
          amounts payable to (I) CSFB for acting as a financial adviser as
          described above, and (II) Fleet, for acting as financial adviser as
          described above in an amount not in excess of $500,000 together with
          reimbursement of reasonable costs and expenses, and (III) all
          severance benefits, accrued vacation or other benefits payable to
          terminated UniSite Employees (other than those paid pursuant to the
          plan referred to in Section 6.14) to the extent they do not, together
          with amounts paid under clause (a)(iii) above or excluded under clause
          (b)(i)(z) above, exceed $2,000,000.


          (c)  There shall be deducted from current assets an amount equal to $5
     million, which was the amount received by UniSite in connection with the
     termination of the GTE Management Agreement.



          (b) If the parties hereafter execute an amended and restated Merger
Agreement and a conflict occurs between this Second Amendment and any amended
and restated Merger Agreement, the Merger Agreement,


as amended by this Second Amendment and any subsequent amendments, shall

control.

     3. Waiver. ATC and ATI hereby agree that any adjustments required by the
conditions to closing set forth in Sections 7.2(d), (f), (j) and (l) of the
Merger Agreement shall be satisfied out of the Merger Consideration Adjustment
in the manner prescribed by Section 3.5 of the Merger Agreement and the failure
of any of such conditions to be satisfied shall not be a reason for ATC and ATI
not to consummate the Merger.

     4. Ratification. All of the terms of the Merger Agreement, as hereby
amended, are ratified and confirmed and shall remain in full force and effect.

     5. Acknowledgment. Each party acknowledges that it has all requisite power
and authority to execute and deliver, and to perform its obligations under, this
Second Amendment.

     6. Miscellaneous.

        (a) If any provision of the Second Amendment is held to be invalid or
unenforceable, in whole or in part, such invalidity or unenforceability shall
not affect any otherwise valid provision, and all other valid provisions shall
remain in full force and effect.

        (b) This Second Amendment may be executed in multiple counterparts,
which taken together shall constitute one and the same document. Facsimile
signatures on this Second Amendment are acceptable in lieu of originals.

        (c) This Second Amendment supersedes all previous agreements,
negotiations, or communications among the parties with respect to the subject
matter hereof, and contains the complete and exclusive expression of the
understandings among the parties with respect to such subject matter. This
Second Amendment cannot be amended, modified, or supplemented in any respect
except by a subsequent written agreement entered into by the parties hereto.

        (d) This Second Amendment will be governed by, and will be construed
according to the laws of the State of New York, without regard to conflict or
choice of law principles of the State of New York that might otherwise cause the
internal laws of any other jurisdiction to be applied.


               IN WITNESS WHEREOF, the parties have executed this Second
Amendment as of the date first written above.


UNISITE, INC.                   AMERICAN TOWER CORPORATION
By:   /s/ Daniel P. Behuniak    By:  /s/ James S. Eisenstein
      ----------------------         -----------------------
Name:   Daniel P. Behuniak      Name:   James S. Eisenstein
       ---------------------           ---------------------
Its:   President and Chief      Its:    Vice President
       Executive Officer               ----------------------
      ----------------------    Date:  December 31, 1999
Date: December 31, 1999                -----------------------
      ----------------------


ATI MERGER CORPORATION
By:  /s/ James S. Eisenstein
     -----------------------
Name:   James S. Eisenstein
       ---------------------
Its:   Vice President
      ----------------------
Date: December 31, 1999
     -----------------------