CONFORMED COPY

                                WAIVER UNDER THE
                           REVOLVING CREDIT AGREEMENT


         WAIVER  dated  as of  December  1,  2000  under  the  Revolving  Credit
Agreement  dated as of March 31, 1998 (as  heretofore  amended,  the  "Revolving
Credit  Agreement")  among MOTIENT  HOLDINGS  INC.  (formerly  AMSC  ACQUISITION
COMPANY,  INC.) (the "Borrower"),  MOTIENT CORPORATION (formerly AMERICAN MOBILE
SATELLITE  CORPORATION) (the "Parent  Guarantor"),  the BANKS party thereto (the
"Banks"), MORGAN GUARANTY TRUST COMPANY OF NEW YORK, as Documentation Agent (the
"Documentation  Agent"),  and TORONTO DOMINION (TEXAS),  INC., as Administrative
Agent.

                              W I T N E S S E T H :

         WHEREAS,  the Parent Guarantor  proposes to enter into the Restructured
Satellite Business Transactions (as defined below);

         WHEREAS,  the  undersigned  Banks and the  Shareholder  Guarantors  are
willing to consent  to the  foregoing  and to waive  certain  provisions  of the
Revolving Credit Agreement in connection with the foregoing;

         NOW, THEREFORE, the undersigned parties hereto agree as follows:

         SECTION  1.  Definitions;  References.  Unless  otherwise  specifically
defined herein,  each term used herein which is defined in the Revolving  Credit
Agreement  has the  meaning  assigned  to  such  term  in the  Revolving  Credit
Agreement. In addition:




         "Restructured  Satellite Business Transactions" means the following and
certain  related  transactions,  each as more fully  described in the Investment
Agreement,  Amended and Restated Motient Services Asset Sale Agreement,  Amended
Research & Development,  Marketing and Service  Agreement,  Limited  Partnership
Agreement,  Amended Registration Rights Agreement,  Amended  Cross-Licensing and
Bulk  Resale   Agreement  and  certain  other  agreements   (collectively,   the
"Transaction  Documents"),  each  substantially in the form delivered to each of
the  Banks  and  Shareholder  Guarantors  prior to  December  1,  2000:  (i) the
conversion of Motient Satellite Ventures LLC into a Delaware limited partnership
("Newco"),  the  Parent  Guarantor's  interest  in  which  may  subsequently  be
transferred to another new  directly-owned  Subsidiary of the Parent  Guarantor,
and the formation of a new corporation to serve as the general partner of Newco,
the ownership of which  corporation  will mirror the economic  interests held in
Newco;  (ii)  the  increase  from  20% to 40% of  the  aggregate  fully  diluted
ownership interests in Newco held by Telcom Ventures,  L.L.C.,  Columbia Capital
Corporation  and Spectrum  Equity  Investors LP or  designated  subsidiaries  or
affiliates thereof (each, a "Purchaser", and collectively, the "Purchasers", and
the  aggregate  amount of such  interests  held by the  Purchasers,  the  "Newco
Purchaser  Interests"),  in  consideration  of the payment by the  Purchasers to
Newco of  $50,000,000,  a portion of which  Newco  Purchaser  Interests,  at the
option of each  Purchaser,  may be  exchanged  for  common  shares of the Parent
Guarantor;  (iii) the  issuance  by Newco to the  Purchasers  of an option  (the
"Newco  Option"),  which may be exercised by one or more of the  Purchasers,  to
purchase an additional 10.66% of the fully diluted ownership interests in Newco,
exercisable  for  $40,000,000  (the "Newco  Option  Exercise  Price");  (iv) the
contribution by TMI Communications and Company,  Limited  Partnership ("TMI") of
substantially  all of its mobile  satellite  business to Newco in exchange for a
cash  payment  of  $7,500,000,  a  note  in  the  initial  principal  amount  of
$11,500,000 (the "TMI Note") and 27.2% of the fully diluted ownership  interests
in Newco, as a result of which the Parent  Guarantor shall own less than half of
the fully  diluted  membership  interests  of Newco and Newco shall  thenceforth
cease to be a  Subsidiary  of the  Parent  Guarantor  or a member of the  Parent
Guarantor Group for all purposes under the Revolving  Credit  Agreement (even if
the Purchasers later exchange a sufficient  amount of Newco Purchaser  Interests
for common shares of the Parent  Guarantor to cause the Parent Guarantor to once
again own more than half of the fully  diluted  membership  interests of Newco);
(v) the execution by Newco and Motient  Services Inc.  (formerly AMSC Subsidiary
Corporation),  a  direct  wholly-owned  Subsidiary  of  the  Borrower  ("Motient
Services"),  of an amended and restated asset sale agreement  pursuant to which,
for a cash  payment  to  Motient  Services  of  $45,000,000  of the  $50,000,000
received from the Purchasers  and the issuance to Motient  Services of a note in
the initial principal amount of $15,000,000 (the "Motient Services Note"),  plus
the  $24,000,000  deposit  previously  paid to  Motient  Services,  Newco  shall
purchase Motient Services'  satellite  communications  services business (either
directly  from  Motient   Services  or  by  purchasing  the  equity  of  a  new,
directly-owned  Subsidiary of Motient Services to which such business shall have
been  contributed),  pending the  consummation of which purchase any proceeds of
In-Orbit  Insurance  shall be held in escrow and upon the  consummation of which
purchase any such escrowed proceeds shall be included in the transferred assets;
(vi) the loan by the Parent Guarantor to Newco of $2,500,000, to be evidenced by
a note in the initial  principal  amount of  $2,500,000,  the  proceeds of which
loan, together with $5,000,000 of the $50,000,000  received from the Purchasers,
will be used to fund the  $7,5000,000  cash payment to TMI (such note,  like the
TMI Note and the Motient  Services Note, to have a stated maturity of five years
and interest  accruable at a rate of 10% per annum, which interest will be added
to principal  semi-annually in arrears,  as well as the other terms set forth in
the Transaction  Documents);  (vii) the formation by the Parent Guarantor of one
or more other  directly-owned  Subsidiaries,  if  necessary  solely to effect an
exchange by the Purchasers of Newco Purchaser Interests for common shares of the
Parent  Guarantor   (whether  through  a  triangular   merger  of  Newco,   such
Subsidiaries  and the Parent  Guarantor or otherwise),  and the merger of one or
more such Subsidiaries with one or more of the Purchasers or their  transferees;
and (viii) the performance by Newco,  Motient Services,  Motient  Communications
Company  and the Parent  Guarantor  of their  respective  obligations  under the
Transaction  Documents (including,  without limitation,  obligations relating to
indemnification,  the sharing and  cross-licensing of assets and the disposition
by  the  Parent  Guarantor  (directly  or  through  a  transaction  involving  a
Subsidiary)  of its remaining  membership  interests in Newco in a  "drag-along"
transaction.

         SECTION 2. Waiver.  The  undersigned  Banks waive  compliance  with the
provisions of Article 5 of the Revolving  Credit Agreement  (including,  without
limitation,  Sections 5.04,  5.12, 5.15, 5.16, 5.18 and 5.19) to the extent (and
only to the extent)  necessary  to permit the  Restructured  Satellite  Business
Transactions. The undersigned Banks agree that the purchase of Motient Services'
satellite  communications  services  business  by Newco  (and  none of the other
Restructured Satellite Business Transactions, with the exception of any transfer
or disposition  by the Parent  Guarantor of any portion of its interest in Newco
or any successor to Newco,  to the extent such transfer or disposition is or may
be contemplated by the Restructured  Satellite  Business  Transactions,  and the
receipt of proceeds of In-Orbit Insurance,  if the purchase of Motient Services'
satellite  communications business by Newco is not consummated) shall constitute
a Reduction Event requiring a reduction of Commitments under Section 2.09(c)(i),
with respect to which (x) the Net Cash Proceeds  shall be the sum of $45,000,000
plus all cash payments  received in respect of the Motient Services Note and (y)
the Reduction Percentage shall be 50%.

         SECTION 3.  Representations  of Borrower and Parent Guarantor.  Each of
the  Borrower  and  Parent  Guarantor  represents  and  warrants  that  (i)  the
representations  and warranties  set forth in Article 4 of the Revolving  Credit
Agreement  shall be true on and as of the  Effective  Date  and (ii) no  Default
shall have occurred and be continuing on such date.

         SECTION  4.  Governing  Law.  This  Waiver  shall be  governed  by and
construed in accordance with the laws of the State of New York.






         SECTION  5.  Counterparts.  This  Waiver may be signed in any number of
counterparts, each of which shall be an original, with the same effect as if the
signatures thereto and hereto were upon the same instrument.

         SECTION 6. Effectiveness.  This Waiver shall become effective as of the
date  hereof on the date (the  "Effective  Date") when the  Documentation  Agent
shall have received a counterpart  hereof from each of the Borrower,  the Parent
Guarantor,  Hughes, SingTel, Baron Capital and the Required Banks signed by such
party or a facsimile or other written  confirmation (in form satisfactory to the
Documentation Agent) that such party has signed a counterpart hereof.

          SECTION 7. Shareholder  Guarantor Consent. The Shareholder  Guarantors
consent to the foregoing.





         IN WITNESS  WHEREOF,  the parties  hereto have caused this Waiver to be
duly executed as of the date first above written.

                                MOTIENT HOLDINGS INC. (formerly
                                AMSC ACQUISITION COMPANY, INC.)


                                By: /s/ Randy S. Segal
                                    -------------------------------------------
                                    Title: Senior Vice President



                                MOTIENT CORPORATION (formerly
                                AMERICAN MOBILE SATELLITE CORPORATION)


                                By: /s/ Randy S. Segal
                                    -------------------------------------------
                                    Title: Senior Vice President



                                TORONTO DOMINION (TEXAS), INC.


                                By: /s/ Lynn Chasin
                                    -------------------------------------------
                                    Title: Vice President








                                MORGAN GUARANTY TRUST COMPANY OF NEW YORK


                                By: /s/ Robert Bottamedi
                                    -------------------------------------------
                                    Title: Vice President

                                BANK OF AMERICA, N.A.


                                By: /s/ Dianne J. Prust
                                    -------------------------------------------
                                    Title: Principal


                                BANCA COMMERCIALE ITALIANA LOS ANGELES FOREIGN
                                 BRANCH


                                By: /s/ Frank Maffei
                                    -------------------------------------------
                                    Title: Vice President


                                By: /s/ Charles Dougherty
                                    -------------------------------------------
                                    Title: Vice President


                                BANCA DI ROMA - SAN FRANCISCO


                                By: /s/ Richard G. Dietz
                                    -------------------------------------------
                                    Title: Vice President


                                By: /s/ Thomas C. Woodruff
                                    -------------------------------------------
                                    Title: Vice President






                                THE CHASE MANHATTAN BANK


                                By: /s/ Tracey Navin Ewing
                                    -------------------------------------------
                                    Title: Vice President




                                CITICORP USA, INC.


                                By: /s/ Walter L. Larsen
                                    -------------------------------------------
                                    Title: Managing Director


                                DEUTSCHE BANK AG, NEW YORK BRANCH AND/OR CAYMAN
                                 ISLANDS BRANCH


                                By: /s/ Virginia Mahler Cosenza
                                    -------------------------------------------
                                    Title: Vice President


                                By: /s/ Andreas Neumeier
                                    -------------------------------------------
                                    Title: Vice President


                                BANK ONE, N.A.


                                By: /s/ Stephen C. Price
                                    -------------------------------------------
                                    Title: First Vice President


                                SAN PAOLO IMI S.P.A.


                                By:
                                    -------------------------------------------
                                    Title:


                                By:
                                    -------------------------------------------
                                    Title:






                                HUGHES ELECTRONICS CORPORATION


                                By: /s/ M. J. Gaines
                                    -------------------------------------------
                                    Title: Corporate Vice President



                                SINGAPORE TELECOMMUNICATIONS LTD.


                                By: /s/ for Tay Chek Khoon
                                    -------------------------------------------
                                    Title: Managing Director
                                           Satellite Business



                                BARON CAPITAL PARTNERS, L.P., a Delaware limited
                                 partnership
                                By: BARON CAPITAL MANAGEMENT, INC., a general
                                    partner


                                By: /s/ Linda S. Martinson
                                    -------------------------------------------
                                    Title: Vice President & General Counsel