<PAGE>

                                                                    EXHIBIT 10.8


                        HUGHES ELECTRONICS CORPORATION


                                FIRST AMENDMENT
               TO REVOLVING CREDIT AGREEMENT (364 DAY FACILITY)

          This FIRST AMENDMENT TO REVOLVING CREDIT AGREEMENT (364-DAY FACILITY)
(this "Amendment") is dated as of December 31, 1999 and entered into by and
among HUGHES ELECTRONICS CORPORATION, a Delaware corporation (the "Borrower"),
the financial institutions listed on the signature pages hereof (the "Banks"),
BANK OF AMERICA, N.A., as the administrative agent for the Banks (in such
capacity the "Administrative Agent"), CITICORP USA, INC. as the syndication
agent (in such capacity the "Syndication Agent") and DEUTSCHE BANK AG, NEW YORK
BRANCH as documentation agent (the "Documentation Agent") and is made with
reference to that certain Revolving Credit Agreement (364-Day Facility) dated as
of November 24, 1999 (as so amended, the "Credit Agreement"), by and among the
Borrower, the lending institutions identified therein, the Administrative Agent,
the Syndication Agent and the Documentation Agent.  Capitalized terms used
herein without definition shall have the same meanings herein as set forth in
the Credit Agreement.

                                   RECITALS

          WHEREAS, Borrower and Banks desire to amend the Credit Agreement to
(i) revise the definition of Consolidated EBITDA;

          NOW, THEREFORE, in consideration of the premises and the agreements,
provisions and covenants herein contained, the parties hereto agree as follows:

     1.   AMENDMENTS TO THE CREDIT AGREEMENT

          1.1  Amendments to Section 1:  Definitions
               -------------------------------------

          Subsection 1.1 of the Credit Agreement is hereby amended by deleting
the definition of Consolidated EBITDA in its entirety, and by replacing it with
the following:

          "Consolidated EBITDA" means, for any period, for Borrower and its
Subsidiaries on a consolidated basis, an amount equal to the sum of (a)
Consolidated Net Income, (b) Consolidated Interest Charges, (c) the amount of
taxes, based on or measured by income, used or included in the determination of
such Consolidated Net Income, (d) the amount of depreciation and amortization
expense deducted in determining such Consolidated Net Income, (e) for each
fiscal quarter in the fiscal year ending December 31, 1999, (i) the amount of
charges taken in connection with the cancellation of contract with Asia Pacific
Mobile Telecommunications, up to an aggregate amount of $92,000,000 for all such
fiscal quarters, and (ii) the amount of charges taken in connection with
development costs and schedule delays at Hughes Space and Communication up to an
aggregate amount of $125,000,000 for all such fiscal quarters, (f) for the last
fiscal quarter of the fiscal year ending December 31, 1999, (i) if a non-cash
charge is taken in connection with the write-off of
<PAGE>

equity investment in ICO Global Communications, the amount of such non-cash
charge up to an aggregate amount of $62,000,000, (ii) if a non-cash charge is
taken in connection with the write-off of equity investment in the Hughes
Network Systems wireless business, the amount of such non-cash charge up to an
aggregate amount of $272,000,000, and (iii) if a change in business strategy
related to DirecTV Japan, Ltd. results in cash and/or non-cash charges, the
amount of such cash and/or non-cash charges up to an aggregate amount of
$250,000,000, and (g) for each fiscal quarter in the fiscal year ending December
31, 2000, the amount of non-cash charges taken in connection with ICO Global
Communications and PanAmSat launch delays, up to an aggregate amount for all
such fiscal quarters of (x) $150,000,000 minus (y) the amount of the charge, if
any, added pursuant to clauses (f)(i) above; minus (h) for each fiscal quarter
                                             -----
in the fiscal year ending December 31, 2000, the amount of cash losses (whether
or not accounted for as charges under GAAP) in connection with DirecTV Japan,
Ltd., but only if such cash losses were reflected in the charges, if any, added
pursuant to clause (f)(iii); plus (i) for each fiscal quarter in the fiscal year
                             ----
ending December 31, 2000, if any change in business strategy regarding DirecTV
Japan, Ltd. results in a non-cash charge, the amount of such non-cash charge up
to an aggregate amount of $150,000,000 minus the amount of non-cash charges, if
any, added pursuant to clause (f)(iii) above.

     2.   CONDITIONS TO EFFECTIVENESS

          Section 1 of this Amendment shall become effective only upon the
satisfaction of all of the following conditions precedent (the date of
satisfaction of such conditions being referred to herein as the "Amendment
Effective Date"):

          (A)  On or before the Amendment Effective Date, Borrower shall deliver
to the Banks (or to the Administrative Agent for the Banks with sufficient
originally executed copies, where appropriate, for each Bank and its counsel)
the following, each, unless otherwise noted, dated the Amendment Effective Date:

          (1)  A certificate, signed by a vice president or assistant treasurer
of Borrower dated the Amendment Effective Date certifying: (i) that the
representations and warranties of Borrower contained in the Credit Agreement are
true and correct in all material respects; and (ii) that, after giving effect to
this Amendment, no event has occurred and is continuing or would result from the
making of a Loan which constitutes or would constitute an Event of Default or an
Unmatured Event of Default; and

          (2)  Copies of this Amendment executed by Borrower, the Majority Banks
and the Administrative Agent.

          (B)  All fees and other amounts due to the Administrative Agent,
Syndication Agent, Documentation Agent, Arranger and any Bank through the
Amendment Effective Date from Borrower shall have been received by such person.

     3.   BORROWER'S REPRESENTATIONS AND WARRANTIES

          In order to induce Banks to enter into this Amendment and to amend the
Credit Agreement in the manner provided herein, Borrower represents and warrants
to each Bank that the following statements are true, correct and complete:

                                       2
<PAGE>

          (A)  Corporate Power and Authority. Borrower has all requisite
               -----------------------------
corporate power and authority to enter into this Amendment and to carry out the
transactions contemplated by, and perform its obligations under, the Credit
Agreement as amended by this Amendment (the "Amended Agreement").

          (B)  Authorization of Agreements.  The execution and delivery of this
               ---------------------------
Amendment and the performance of the Amended Agreement have been duly authorized
by all necessary corporate action on the part of Borrower.

          (C)  No Contravention.  There is no charter, by-law, or capital stock
               ----------------
provision of Borrower and no provision of any indenture or material agreement,
written or oral, to which Borrower is a party or under which Borrower is
obligated, nor is there any statute, rule or regulation, or any judgment, decree
or order of any court or agency binding on Borrower which would be contravened
by the execution, delivery and performance of any provision, condition, covenant
or other term of this Amendment or the Amended Agreement.

          (D)  Binding Obligation. This Amendment and the Amended Agreement are
               ------------------
the legal, valid and binding obligation of Borrower, enforceable against it in
accordance with their terms, and any instrument or agreement required hereunder
or by the Amended Agreement, when executed and delivered, will be similarly
valid, binding and enforceable.

          (E)  Incorporation of Representations and Warranties From Credit
               -----------------------------------------------------------
Agreement. The representations and warranties contained in Section 6 of the
---------
Credit Agreement are and will be true, correct and complete in all material
respects on and as of the Amendment Effective Date to the same extent as though
made on and as of that date, except to the extent such representations and
warranties specifically relate to an earlier date, in which case they were true,
correct and complete in all material respects on and as of such earlier date.

          (F)  Absence of Default. After giving effect to this Amendment, no
               ------------------
event has occurred and is continuing or will result from the consummation of the
transactions contemplated by this Amendment that would constitute an Event of
Default or a Unmatured Event of Default.

     4.   MISCELLANEOUS

          (A)  Reference to and Effect on the Credit Agreement and the Other
Loan Documents.

          (i)  On and after the Amendment Effective Date, each reference in the
Credit Agreement to "this Agreement", "hereunder", "hereof", "herein" or words
of like import referring to the Credit Agreement, and each reference in the
other documents entered pursuant to the Credit Agreement to the "Credit
Agreement", "thereunder", "thereof" or words of like import referring to the
Credit Agreement shall mean and be a reference to the Amended Agreement.

                                       3
<PAGE>

          (ii)  Except as specifically amended by this Amendment, the Credit
Agreement and the other documents entered pursuant to the Credit Agreement shall
remain in full force and effect and are hereby ratified and confirmed.

          (iii) The execution, delivery and performance of this Amendment shall
not, except as expressly provided herein, constitute a waiver of any provision
of, or operate as a waiver of any right, power or remedy of the Administrative
Agent or any Bank under, the Credit Agreement or any of the other Loan
Documents.

          (B)   Fees and Expenses. Borrower acknowledges that all costs, fees
                -----------------
and expenses as described in subsection 11.14 of the Credit Agreement incurred
by the Arranger, the Administrative Agent and their counsel with respect to this
Amendment and the documents and transactions contemplated hereby shall be for
the account of Borrower.

          (C)   Headings.  Section and subsection headings in this Amendment are
                --------
included herein for convenience of reference only and shall not constitute a
part of this Amendment for any other purpose or be given any substantive effect.

          (D)   California Law. The interpretation, enforcement and effect of
                --------------
this Amendment shall be governed and controlled in all respects by and construed
according to the substantive laws of the State of California.

          (E)   Counterparts; Effectiveness. This Amendment may be executed in
                ---------------------------
any number of counterparts and by different parties hereto in separate
counterparts, each of which when so executed and delivered shall be deemed an
original, but all such counterparts together shall constitute but one and the
same instrument; signature pages may be detached from multiple separate
counterparts and attached to a single counterpart so that all signature pages
are physically attached to the same document. This Amendment (other than the
provisions of Section 1 hereof, the effectiveness of which is governed by
Section 2 hereof) shall become effective upon the execution of a counterpart
hereof by Borrower and Majority Banks and receipt by Borrower and the
Administrative Agent of written or telephonic notification of such execution and
authorization of delivery thereof.

                 [Remainder of page intentionally left blank]

                                       4
<PAGE>

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

                                        HUGHES ELECTRONICS CORPORATION



                                        By: _______________________________
                                        Name:
                                        Title:

                                      S-1
<PAGE>

                                        BANK OF AMERICA, N.A., as
                                        Administrative Agent



                                        By: ______________________________
                                        Name:
                                        Title:

                                      S-2
<PAGE>

                                        CITICORP USA, INC., as Syndication
                                        Agent and as a Bank



                                        By: ________________________________
                                        Name:
                                        Title:

                                      S-3
<PAGE>

                                        DEUTSCHE BANK AG, NEW YORK BRANCH,
                                        as Documentation Agent



                                        By: ________________________________
                                        Name:
                                        Title:

                                      S-4
<PAGE>

                                        ALLFIRST BANK


                                        By:___________________________________
                                        Name:
                                        Title:

                                      S-5
<PAGE>

                                   BANCA DI ROMA - SAN FRANCISCO

                                   By:________________________________________
                                   Name:
                                   Title:

                                      S-6
<PAGE>

                                   BANK OF AMERICA, N.A., as a Bank

                                   By:________________________________________
                                   Name:
                                   Title:

                                      S-7
<PAGE>

                                   THE BANK OF NEW YORK

                                   By:________________________________________
                                   Name:
                                   Title:

                                      S-8
<PAGE>

                                   BANK ONE, NA

                                   By:________________________________________
                                   Name:
                                   Title:

                                      S-9
<PAGE>

                                   BARCLAYS BANK PLC

                                   By:________________________________________
                                   Name:
                                   Title:

                                     S-10
<PAGE>

                                   BANQUE NATIONALE DE PARIS

                                   By:________________________________________
                                   Name:
                                   Title:

                                   By:________________________________________
                                   Name:
                                   Title:

                                     S-11
<PAGE>

                                   BAYERISCHE LANDESBANK
                                   GIROZENTRALE, CAYMAN ISLANDS
                                   BRANCH

                                   By:________________________________________
                                   Name:
                                   Title:

                                     S-12
<PAGE>

                                   CREDIT SUISSE FIRST BOSTON


                                   By:________________________________________
                                   Name:
                                   Title:

                                   By:________________________________________
                                   Name:
                                   Title:

                                     S-13
<PAGE>

                                   DEUTSCHE BANK AG, NEW YORK
                                   BRANCH AND/OR CAYMAN ISLAND
                                   BRANCHES, as a Bank

                                   By:________________________________________
                                   Name:
                                   Title:

                                   By:________________________________________
                                   Name:
                                   Title:

                                     S-14
<PAGE>

                                   THE FUJI BANK, LIMITED

                                   By:________________________________________
                                   Name:
                                   Title:

                                     S-15
<PAGE>

                                   MELLON BANK, N.A.


                                   By:________________________________________
                                   Name:
                                   Title:

                                     S-16
<PAGE>

                                   THE MITSUBISHI TRUST AND BANKING
                                   CORPORATION, NEW YORK BRANCH


                                   By:________________________________________
                                   Name:
                                   Title:

                                     S-17
<PAGE>

                                   MORGAN GUARANTY TRUST
                                   COMPANY OF NEW YORK


                                   By:________________________________________
                                   Name:
                                   Title:

                                     S-18
<PAGE>

                                   SOCIETE GENERALE


                                   By:________________________________________
                                   Name:
                                   Title:

                                     S-19
<PAGE>

                                   WESTDEUTSCHE LANDESBANK
                                   GIROZENTRALE, NEW YORK BRANCH


                                   By:________________________________________
                                   Name:
                                   Title:


                                   By:________________________________________
                                   Name:
                                   Title:

                                     S-20