Execution Copy


                              ASSUMPTION, RELEASE,
                         AMENDMENT AND WAIVER AGREEMENT


         This  Assumption,   Release,   Amendment  and  Waiver  Agreement  (this
"Agreement")  is entered  into as of December  29,  2000 by and among  Motorola,
Inc.,  a Delaware  corporation  ("Motorola"),  Motient  Communications  Inc.,  a
Delaware  corporation  ("You" or the "New Borrower") and Motient  Communications
Company (formerly known as ARDIS Company),  a New York general  partnership (the
"Existing Borrower").

                                    RECITALS

     A.  Motorola and the Existing  Borrower are parties to that certain  Credit
Agreement  dated as of June 17, 1998,  as amended by  Amendment  No. 1 to Credit
Agreement dated as of October 15, 1998, as further amended by Amendment No. 2 to
Credit  Agreement  dated as of September 1, 2000 (as so amended,  the  "Original
Credit  Agreement"  and as  further  amended  by  this  Agreement,  the  "Credit
Agreement").  Capitalized  terms used herein without  definition  shall have the
meanings ascribed to them in the Credit Agreement;

     B. In connection with a contemplated corporate  restructuring,  the parties
hereto have advised  Motorola that (a) the Existing  Borrower  intends to assign
all of its  assets  and  liabilities  on a pro  rata  basis  to its two  general
partners, New Borrower and Motient Partner Inc., a Delaware corporation ("MPI"),
and  thereafter  dissolve  and  wind up its  affairs  and (b)  contemporaneously
therewith,  MPI  intends to merge with and into the New  Borrower,  with the New
Borrower  surviving the merger (such  contemplated  corporate  restructuring  as
described to Motorola by the parties hereto and in documents previously provided
to Motorola, the "Restructuring");

     C. The Restructuring is not permitted under the terms and conditions of the
Original Credit Agreement;

     D. The parties  hereto have  requested  that  Motoroloa  (a) waive  certain
provisions of the Original Credit  Agreement in order to permit the consummation
of the  Restructuring;  (b)  amend the  Original  Credit  Agreement  to join New
Borrower  as a  "Borrower"  under the  Credit  Agreement;  and (c)  release  the
Existing Borrower from its liabilities and obligations as a "Borrower" under the
Credit Agreement; and

     E. Motorola is willing to agree to such requests, in each case on the terms
and subject to the conditions set forth herein;

     NOW, THEREFORE, in consideration of the foregoing and the mutual agreements
contained herein, and subject to the terms and conditions hereof, the respective
parties hereto hereby agree as follows:

1. Joinder and Assumption. Effective as of the date hereof, upon satisfaction of
the conditions  precedent set forth in Section 6 below, and in reliance upon the
representations  and  warranties of the New Borrower set forth herein,  Motorola
and the New  Borrower  hereby  acknowledge  and agree  that,  by New  Borrower's
execution and delivery hereof,  without any further notice or action of any kind
whatsoever on its part or on the part of any other person that:

(a)  for all  purposes,  the New Borrower is hereby  joined  automatically  as a
     "Borrower" party to the Credit  Agreement,  being hereafter bound by all of
     the respective terms and provisions thereof; and

(b)  New Borrower assumes and becomes liable for the prompt payment,  observance
     and performance of all Obligations,

     in the case of both  clause (a) and (b) above,  to the same extent and with
     the same force and effect as if New Borrower  had been a  "Borrower"  under
     and an original signatory to the Credit Agreement.

2.  Release  of  Existing  Borrower.  Effective  as of  the  date  hereof,  upon
satisfaction of the conditions precedent set forth in Section 6 below,  Motorola
hereby releases the Existing Borrower from all obligations and liabilities under
the  Credit  Agreement  and  the  Existing  Borrower  shall  thereafter  have no
liability or obligations under the Credit Agreement.

3.  Amendments  to  Credit  Agreement.  Effective  as of the date  hereof,  upon
satisfaction  of the  conditions  precedent  set forth in  Section 6 below,  all
references  to  "Motient  Communications  Company" in the Credit  Agreement  are
hereby amended and deemed to be references to "Motient Communications Inc."

4.   Consent and Waiver.

(a)  Motorola hereby consents to the  Restructuring  and to the  consummation of
     the actions contemplated thereby,  including the assignment by the Existing
     Borrower  of all of its assets and  liabilities  on a pro rata basis to the
     New Borrower and MPI; provided,  however, that each of the New Borrower and
     MPI shall take such assets subject to Motorola's  security interest therein
     as  perfected  against  the  Existing  Borrower  and the  liquidiation  and
     dissolution of the Existing Borrower.

(b)  Any breach of the  provisions  of Sections  4.4,  5.2 and 9.6 of the Credit
     Agreement arising as a result of the Restructuring is hereby waived.

5.  Representations  and  Warranties.  In order to induce Motorola to enter into
this Agreement,  the New Borrower  hereby  represents to Motorola the following,
except as  otherwise  disclosed  to  Motorola in writing  concurrently  with the
execution of this Agreement:

                  (a) Status.  You are duly incorporated or formed,  and validly
         existing under the laws of the state of incorporation or formation. You
         (a) have the power and authority and the legal right to own and operate
         your  property,  to lease the property you operate,  and to conduct the
         business in which you are currently engaged and in which you propose to
         engage,  (b) are in compliance  with all  Requirements of Law except to
         the extent  that the  failure  to comply  therewith  could not,  in the
         aggregate, have a material adverse effect on your business, operations,
         assets (taken in the aggregate) or financial  condition,  and could not
         materially  adversely  affect your ability to perform your  obligations
         under this Agreement or the Credit  Agreement and (c) have qualified to
         do  business  in all  jurisdictions  where  your  ownership,  lease  or
         operation  of property or the conduct of your  business  requires  such
         qualification or recording, except to the extent that the failure to so
         qualify could not, in the aggregate,  have a material adverse effect on
         your business, operations, assets (taken in the aggregate) or financial
         condition,  and could not materially  adversely  affect your ability to
         perform your obligations under this Agreement and the Credit Agreement.

                  (b) Power and  Authority.  You have the power,  authority  and
         legal right to execute,  deliver and  perform  this  Agreement  and the
         Credit  Agreement  and to borrow under the Credit  Agreement,  and have
         taken all  necessary  action to  authorize  the Credit on the terms and
         conditions  of the Credit  Agreement  and to authorize  the  execution,
         delivery and performance of this Agreement and the Credit Agreement and
         the  related  documents   described  herein  and  therein.   Where  any
         Governmental Authority,  including without limitation any PUC, requires
         consents, filings or authorizations prior to the Credit, you shall have
         obtained all such consents, filings or authorizations.  Other than such
         consents,  filings or  authorizations,  no consent or  authorization or
         filing  with,  or  other  act by or with  respect  to any  Governmental
         Authority,  is required in connection  with the Credit under the Credit
         Agreement or with the  execution,  delivery,  performance,  validity or
         enforceability  of  this  Agreement  or the  performance,  validity  or
         enforceability  of the Credit  Agreement.  This Agreement has been duly
         executed and  delivered  and this  Agreement  and the Credit  Agreement
         constitutes your legal, valid and binding obligation,  which obligation
         shall be  enforceable  against you in accordance  with the terms of the
         Agreement  or the  Credit  Agreement,  as the  case may be,  except  as
         enforceability  may be limited by  applicable  bankruptcy,  insolvency,
         reorganization, moratorium or similar laws affecting the enforcement of
         creditors' rights generally and general equitable principles.

                  (c) No Violations. The execution,  delivery and performance of
         the  Agreement  and the use of the  proceeds of the Credit (i) will not
         violate,  be in conflict  with,  result in a breach of or  constitute a
         default  under,  any  Requirement  of Law or  any of  your  contractual
         obligations,  except to the extent such  violations,  in the aggregate,
         could  not  have  a  material  adverse  effect  on (a)  your  business,
         operations,  assets (taken in the aggregate) or financial condition, or
         (b) your ability to perform your obligations under the Agreement or the
         Credit Agreement, and (ii) will not result in, or require, the creation
         or  imposition  of any  Lien  on any of  your  properties  or  revenues
         pursuant to any  Requirement  of Law or contractual  obligation,  other
         than  pursuant to the  Agreement  or the Credit  Agreement.  You are in
         compliance with the Employee  Retirement Income Security Act of 1974 as
         amended from time to time  (ERISA),  and neither the  execution nor the
         performance  of  the  Agreement  nor  the  performance  of  the  Credit
         Agreement  by you will result in any  violation  of ERISA.  Any benefit
         plan  that is  subject  to ERISA  has been  properly  accounted  for in
         Financial Statements attached to the Credit Agreement.

                  (d)  No  Pending  Actions.  No  litigation,  investigation  or
         proceedings  of or before any arbitrator or  Governmental  Authority is
         pending,  or, to your knowledge is threatened,  against you or, against
         any of your properties or revenues (a) with respect to the Agreement or
         any  of  the  transactions   contemplated  thereby,  or  (b)  which  is
         reasonably expected to be adversely determined, and which, if adversely
         determined,  could,  individually or in the aggregate,  have a material
         adverse  effect  on your  business,  operations,  assets  (taken in the
         aggregate) or financial condition.

                  (e) No Defaults.  You are not in default under or with respect
         to any  contractual  obligation  where such default could be materially
         adverse to your business,  operations,  assets (taken in the aggregate)
         or financial condition,  or which could materially and adversely affect
         your ability to perform  your  obligations  under the  Agreement or the
         Credit  Agreement.  No Default or Event of Default has  occurred and is
         continuing.

                  (f) Good  Title.  Any of your  leases  are in full  force  and
         effect, and you enjoy peaceful and undisturbed  possession  thereunder;
         you  have a  recorded  title  in fee  simple  to all  your  owned  real
         property,  and good and  marketable  title to all your  other  personal
         property.

                  (g) Taxes.  You have filed or caused to be filed all  material
         tax returns  which are  required by law to be filed,  and have paid all
         taxes shown to be due and payable on said returns or on any  assessment
         made by any  Governmental  Authority  (other  than  those the amount or
         validity  of  which  is  currently  being  contested  in good  faith by
         appropriate   proceedings   and  with  respect  to  which  reserves  in
         conformity  with GAAP have been  provided  on your  books);  and no tax
         Liens  have been  filed  and,  to your  knowledge,  no claims are being
         asserted  with respect to any such taxes,  fees or other  charges other
         than inchoate Liens for taxes not yet due.

                  (h) No Extending of Credit.  You have not engaged nor will you
         generally  engage in the business of purchasing or selling Margin Stock
         (as defined in  Regulation G, T, U or X of the Board of Governors or of
         the  Federal  Reserve  System)  extending  credit  for the  purpose  of
         purchasing Margin Stock.

                  (i)  No Subsidiaries.   You  have  no  subsidiaries and do not
         control, directly or indirectly, any other business entity.

                  (j) Patents, Trademarks, etc. You own or have the right to use
         all of the patents,  trademarks,  permits,  service marks, trade names,
         copyrights,  licenses  and  franchises  or rights  with  respect to the
         foregoing (collectively  "patents"),  necessary for the conduct of your
         business as presently contemplated, without any known conflict with the
         rights of others.

                  (k) Information,  Reports,  etc. All information,  reports and
         other  papers and data  furnished  to Motorola by you on or at any time
         after  the  date  hereof  are or will  be,  at the time the same are so
         furnished,  complete  and  correct in all  material  respects;  and all
         projections concerning your business furnished by you, as supplemented,
         will  be  prepared  or  presented  in  good  faith  by you  and  have a
         reasonable  basis.  No  fact  is  known  to you  which  materially  and
         adversely  affects or in the  future may (so far as you can  reasonably
         foresee)  materially  and adversely  affect the  business,  operations,
         assets  (taken as a whole) or your  financial  condition  which has not
         been set  forth in the  Financial  Statements  or in such  information,
         reports, papers and data.

                  (l) Security Documents. The provisions of the Credit Agreement
         are  effective  to  create  in favor of  Motorola  a legal,  valid  and
         enforceable security interest in all of the New Borrower's right, title
         and  interest in the  Collateral  in which a security  interest  may be
         created under Article 9 of the Uniform  Commercial  Code;  and when (i)
         financing   statements   have  been   filed  in  the   offices  in  the
         jurisdictions  listed in Exhibit "E" to the Credit Agreement,  and (ii)
         except  for any  further  filing or taking of  possession  which may be
         required  under Section 9-306 of the UCC in order to perfect a security
         interest  in proceeds of the  Collateral  and any taking of  possession
         which  may be  required  under the UCC in order to  perfect a  security
         interest in instruments,  the Credit  Agreement will create and grant a
         fully perfected first Lien on, and security  interest in the Collateral
         (including  proceeds)  in which a security  interest  may be  perfected
         under Article 9 of the UCC.

                  (m) Governmental Regulation.  You hold sufficient FCC licenses
         for the conduct of your  business  in each area in which you  currently
         conduct your business. All of such licenses are valid,  uncontested and
         in full force and effect.

                  (n) Assumed   Names.   You  are not doing  business  under any
         fictitious  or assumed names, except as disclosed in Exhibit "F" to the
         Credit Agreement.

                  (o) Principal Place of Business.  Your chief executive  office
         and principal place of business are located at the notice address shown
         next to your signature block below. Your books and records with respect
         to the Collateral are kept at this address.

                  (p) Environmental and Safety Matters. You are in compliance in
         all  material  respects  with  all  federal,  state,  local  and  other
         statutes,   ordinances,  orders,  judgments,  rulings  and  regulations
         relating  to the  environment,  environmental  regulation  or  control,
         employee health and safety, or the generation,  use, storage,  disposal
         or transportation of toxic or hazardous materials, substances or wastes
         (collectively, "Environmental Laws").

                  (q)  Collateral.  You  represent  that  each of the  items  of
         Collateral  delivered prior to the date of this Agreement is identified
         by serial number in Exhibit D to this  Agreement,  and is located as of
         the date of this Agreement in the jurisdiction listed in Exhibit D.

6.  Conditions to  Effectiveness.  This Agreement  shall become  effective,  and
Motorola's  commitment  to provide  the Credit to you as set forth in the Credit
Agreement shall be conditioned  upon, the  satisfaction of each of the following
conditions precedent:

(a)      No Default or Event of Default shall have  occurred and be  continuing,
         and you shall have  delivered  to  Motorola a  certificate  of Randy S.
         Segal, Senior Vice President and General Counsel, to such effect.

(b)      You shall have  delivered  to Motorola  executed  counterparts  of this
         Agreement,  and an  executed  original  of  the  Amended  and  Restated
         Promissory Note in the form attached hereto as Exhibit A (the "Note").

(c)      You shall have  delivered to Motorola a certificate of the Secretary or
         other  authorized  officer of the New Borrower as to resolutions of its
         Board of Directors authorizing the execution,  delivery and performance
         by the New  Borrower  of  this  Agreement,  the  Note,  and  the  other
         documents  and   instruments  to  be  delivered  by  the  New  Borrower
         hereunder.

(d)      You  shall have delivered to Motorola an Amended and Restated Guarantee
         Agreement  of each of Motient  Corporation  (formerly  American  Mobile
         Satellite  Corporation)  and  Motient  Holdings  Inc.  (formerly  AMSC
         Acquisition  Company,  Inc.),  each in the  form  attached  hereto  as
         Exhibit B  and Exhibit C respectively.


(e)      You shall have  delivered to Motorola a certificate of the Secretary or
         other authorized officer of each of the Guarantors as to resolutions of
         the Board of Directors of such  Guarantor  authorizing  the  execution,
         delivery and  performance by such Guarantor of the Amended and Restated
         Guarantee  Agreement  to be  delivered  by it  hereunder  and the other
         documents  and   instruments  to  be  delivered  by  the  New  Borrower
         hereunder.

(f)      You shall have  delivered  to Motorola a  certificate  or  certificates
         certifying  the  incumbency of the officer or officers  executing  this
         Agreement, the Note, each Amended and Restated Guarantee Agreement, and
         each other document and instrument delivered hereunder.

(g)      You  shall   have   delivered   to   Motorola  an  opinion  of  counsel
         satisfactory  to   Motorola  as to (i) the due  organization  and valid
         existence of each  of the New Borrower and each Guarantor, (ii) the due
         authorization,   execution,  delivery and  enforceability of the Credit
         Agreement,  as  amended by the  Agreement,  the Note,  the  Amended and
         Restated Guarantees,  and  each other document and instrument delivered
         hereunder,  (iii)  the  absence  of  any  violation  or breach of other
         agreements or applicable Laws, (iv)  the absence of material litigation
         pending or threatened against the New  Borrower and each Guarantor, and
         (v) and as to such other matters as  Motorola shall reasonably request,
         including  without   limitation,    that  the  Restructuring  has  been
         completed  and as a result  title  to  Collateral  financed  under  the
         Credit Agreement has been legally transferred to New Borrower.

(h)      You  shall  have  delivered  each  other  document  and  instrument  as
         Motorola shall have reasonably requested.

Motorola's  obligation  to fund  Advances  under the Credit  Agreement  shall be
subject  to each of the  conditions  set  forth in  Section  8 of the  Terms and
Conditions.

7.  Ratification  of Credit  Agreement.  Except as amended  hereby,  the parties
hereto hereby ratify and confirm the Credit Agreement,  and all of the terms set
forth or incorporated therein.

8. Governing Law. This Agreement will be governed by the law of Illinois without
regard to its conflicts of law rules.

9.  Counterparts.  This  Agreement  may be executed in one or more  counterparts
which, taken together, shall constitute one and the same instrument.

                                             [signature page follows]







Executed by the parties hereto as of the date first set forth above.


                                       MOTIENT COMMUNICATIONS COMPANY


                                       By: /s/ Richard J. Burnheimer
                                           ------------------------------
                                       Its: Vice President and Treasurer


                                       MOTIENT COMMUNICATIONS INC.


                                       By: /s/ Richard J. Burnheimer
                                           -----------------------------
                                       Its: Vice President and Treasurer


                                       MOTOROLA, INC.


                                       By: /s/ Walter Keating
                                           -----------------------------
                                       Its: Vice President







                                    Exhibit A

                                  Form of Note






                                    Exhibit B

                           Form of Motient Corporation
                         Amended and Restated Guarantee





                                    Exhibit C

                          Form of Motient Holdings Inc.
                         Amended and Restated Guarantee





                                    Exhibit D

                                   Collateral