<PAGE>

                                Exhibit 10.29.

    Seventh Amendment to the Loan Agreement with Chase Bank of Texas, N.A.
<PAGE>

                                                                   Exhibit 10.29


                      SEVENTH AMENDMENT TO LOAN AGREEMENT
                      -----------------------------------


     This SEVENTH AMENDMENT TO LOAN AGREEMENT (this "Amendment"). dated as of
                                                     -----------
January 28, 2000, is among SONIC CORP., a Delaware corporation (the "Borrower"),
                                                                     --------
each of the banks or other lending institutions which is or may from time to
time become a signatory or party to the Agreement (hereinafter defined) or any
successor or permitted assignee thereof (each a "Bank" and collectively, the
                                                 ----
"Banks"), and CHASE BANK OF TEXAS, NATIONAL ASSOCIATION (formerly known as Texas
------
Commerce Bank National Association), a national banking association ("Chase"),
                                                                      -----
as agent for itself and the other Banks and as issuer of Letters of Credit under
the Agreement (in such capacity, together with its successors in such capacity,
the "Agent").
     -----

                                   RECITALS:

     A.  Borrower, Agent and Banks have entered into that certain Loan Agreement
dated as of July 12, 1995, as amended by (i) that certain First Amendment to
Loan Agreement dated as of August 16, 1996, (ii) that certain Second Amendment
to Loan Agreement dated as of September 27, 1996, (iii) that certain Third
Amendment to Loan Agreement dated as of June 19, 1997, (iv) that certain Fourth
Amendment to Loan Agreement dated as of January 27, 1998, (v) that certain Fifth
Amendment to Loan Agreement dated as of April 2, 1998, and (vi) that certain
Sixth Amendment to Loan Agreement dated as of June 2, 1999 (as amended, the
"Agreement").
----------

     B.  Pursuant to the Agreement, the undersigned guarantors (each a
"Guarantor" and, collectively, the "Guarantors") have executed Guaranties which
----------                          ----------
guarantee to Agent the payment and performance of the Obligations.

     C.  Borrower, Agent and Banks now desire to amend the Agreement to extend
the Termination Date, as provided herein.

     NOW, THEREFORE, in consideration of the premises herein contained and other
good and valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties hereto agree as follows:

                                   ARTICLE I

                                  Definitions
                                  -----------

     Section 1.1  Definitions. Capitalized terms used in this Amendment, to the
                  -----------
extent no otherwise defined herein, shall have the same meanings as in the
Agreement, as amended hereby.
<PAGE>

                                   ARTICLE II

                                   Amendments
                                   ----------

     Section 2.1  Amendment to Termination Date. Effective as of the date
                  -----------------------------
hereof, the definition of "Termination Date" appearing in Section 1.1 of the
Agreement is hereby amended to read in its entirety as follows:

          "Termination Date" means the earlier of (a) 11:00 a.m. Dallas, Texas
           ----------------
     time on July 12, 2002 or such later date to which the Termination Date may
     be extended pursuant to Section 2.11, or (b) such earlier date on which the
     Commitments terminate as provided in this Agreement.


                                  ARTICLE III

                              Conditions Precedent
                              --------------------

     Section 3.1  Conditions. The effectiveness of this Amendment is subject to
                  ----------
the satisfaction of each of the following conditions precedent:

     (a) Representations and Warranties. The representations and warranties
         ------------------------------
contained herein and in all other Loan Documents, as amended hereby, shall be
true and correct as of the date hereof as if made on the date hereof;

     (b) No Default. No Event of Default shall have occurred and be continuing
         ----------
and no event or condition shall have occurred that with the giving of notice or
lapse of time or both would be an Event of Default; and

     (c) Corporate Matters. All corporate proceedings taken in connection with
         -----------------
the transactions contemplated by this Amendment and all documents, instruments,
and other legal matters incident thereto shall be satisfactory to Agent and its
legal counsel, Winstead Sechrest & Minick P.C.

                                   ARTICLE IV

                 Ratifications, Representations and Warranties
                 ---------------------------------------------

  Section 4.1  Ratifications. The terms and provisions set forth in this
               -------------
Amendment shall modify and supersede all inconsistent terms and provisions set
forth in the Agreement and except
<PAGE>

as expressly modified and superseded by this Amendment, the terms and provisions
of the Agreement are ratified and confirmed and shall continue in full force and
effect. Borrower, Agent and the Banks agree that the Agreement as amended hereby
shall continue to be legal, valid, binding and enforceable in accordance with
its terms.

     Section 4.2  Release of Claims. The Borrower and the Guarantors each hereby
                  -----------------
acknowledge and agree that to their knowledge none of them has any and there are
no claims or offsets against or defenses or counterclaims to the terms and
provisions of or the obligations of the Borrower, any Guarantor or any
Subsidiary created or evidenced by the Agreement or any of the other Loan
Documents, and to the extent any such claims, offsets, defenses or counterclaims
exist, the Borrower and the Guarantors each hereby waive, and hereby release the
Agent and each of the Banks from, any and all claims, offsets, defenses and
counterclaims that are known to the Borrower or any Guarantor as of the date
hereof, such waiver and release being with full knowledge and understanding of
the circumstances and effects of such waiver and release and after having
consulted legal counsel with respect thereto.

     Section 4.3  Representations and Warranties. Borrower hereby represents and
                  ------------------------------
warrants to Agent and the Banks that (i) the execution, delivery and performance
of this Amendment and any and all other Loan Documents executed and/or delivered
in connection herewith have been authorized by all requisite corporate,
partnership and trust action on the part of Borrower and the Guarantors and will
not violate the articles of incorporation, bylaws, partnership agreement or
other organizational documents of Borrower or the Guarantors, (ii) the
representations and warranties contained in the Agreement, as amended hereby,
and any other Loan Document are true and correct on and as of the date hereof as
though made on and as of the date hereof, (iii) no Event of Default has occurred
and is continuing and no event or condition has occurred that with the giving of
notice or lapse of time or both would be an Event of Default, and (iv) Borrower
is in full compliance with all covenants and agreements contained in the
Agreement as amended hereby.

                                   ARTICLE V

                                 Miscellaneous
                                 -------------

     Section 5.1  Survival of Representations and Warranties. All
                  ------------------------------------------
representations and warranties made in this Amendment or any other Loan Document
including any Loan Document furnished in connection with this Amendment shall
survive the execution and delivery of this Amendment and the other Loan
Documents, and no investigation by Agent or any Bank or any closing shall affect
the representations and warranties or the right of Agent and the Banks to rely
upon them.

     Section 5.2  Reference to Agreement. Each of the Loan Documents, including
                  ----------------------
the Agreement and any and all other agreements, documents, or instruments now or
hereafter executed and delivered pursuant to the terms hereof or pursuant to the
terms of the Agreement as
<PAGE>

amended hereby, are hereby amended so that any reference in such Loan Documents
to the Agreement shall mean a reference to the Agreement as amended hereby.

     Section 5.3  Expenses of Agent. As provided in the Agreement, Borrower
                  -----------------
agrees to pay on demand all reasonable costs and expenses incurred by Agent in
connection with the preparation, negotiation, and execution of this Amendment
and the other Loan Documents executed pursuant hereto and any and all
amendments, modifications, and supplements thereto, including without limitation
the reasonable costs and fees of Agent's legal counsel.

     Section 5.4  Severability. Any provision of this Amendment held by a court
                  ------------
of competent jurisdiction to be invalid or unenforceable shall not impair or
invalidate the remainder of this Amendment and the effect thereof shall be
confined to the provision so held to be invalid or unenforceable.

     Section 5.5  APPLICABLE LAW. THIS AMENDMENT AND ALL OTHER LOAN DOCUMENTS
                  --------------
EXECUTED PURSUANT HERETO SHALL BE DEEMED TO HAVE BEEN MADE AND TO BE PERFORMABLE
IN DALLAS, DALLAS COUNTY, TEXAS AND SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF TEXAS.

     Section 5.6  Successors and Assigns. This Amendment is binding upon and
                  ----------------------
shall inure to the benefit of Borrower, Agent and the Banks and their respective
successors and permitted assigns, except Borrower may not assign or transfer any
of its rights or obligations hereunder without the prior written consent of
Agent.

     Section 5.7  Counterparts. This Amendment may be executed in one or more
                  ------------
counterparts, each of which when so executed shall be deemed to be an original,
but all of which when taken together shall constitute one and the same
instrument.
<PAGE>

     Section 5.8   Effect of Waiver. No consent or waiver, express or implied,
                   ----------------
by Agent or any Bank to or for any breach of or deviation from any covenant,
condition or duty by Borrower or any Guarantor shall be deemed a consent or
waiver to or of any other breach of the same or any other covenant, condition or
duty.

     Section 5.9   Headings. The headings, captions, and arrangements used in
                   --------
this Amendment are for convenience only and shall not affect the interpretation
of this Amendment.

     Section 5.10  Non-Application of Chapter 346 of Texas Finance Code. The
                   ----------------------------------------------------
provisions of Chapter 346 of the Texas Finance Code (formerly Chapter 15 of the
Texas Credit Code (Vernon's Annotated Texas Statutes, Article 5069-15)), as
amended, are specifically declared by the parties not to be applicable to this
Amendment or any of the Loan Documents or the transactions contemplated hereby.

     Section 5.11  ENTIRE AGREEMENT. THIS AMENDMENT AND ALL OTHER INSTRUMENTS,
                   ----------------
DOCUMENTS AND AGREEMENTS EXECUTED AND DELIVERED IN CONNECTION WITH THIS
AMENDMENT EMBODY THE FINAL, ENTIRE AGREEMENT AMONG THE PARTIES HERETO REGARDING
THIS AMENDMENT AND SUPERSEDE ANY AND ALL PRIOR COMMITMENTS, AGREEMENTS,
REPRESENTATIONS AND UNDERSTANDINGS, WHETHER WRITTEN OR ORAL, RELATING TO THIS
AMENDMENT, AND MAY NOT BE CONTRADICTED OR VARIED BY EVIDENCE OF PRIOR,
CONTEMPORANEOUS OR SUBSEQUENT ORAL AGREEMENTS OR DISCUSSIONS OF THE PARTIES
HERETO. THERE ARE NO ORAL AGREEMENTS AMONG THE PARTIES HERETO.

     Executed as of the date first written above.

                                   BORROWER:
                                   --------

                                  SONIC CORP.


                             By: /s/ W. Scott McLain
                                --------------------------------
                                W. Scott McLain
                                Chief Financial Officer
<PAGE>

                         AGENTS AND BANKS:
                         -----------------

                         CHASE BANK OF TEXAS, NATIONAL
                         ASSOCIATION (formerly known as Texas
                         Commerce Bank National Association), as Agent
                         and as a Bank.


                         By:
                         /s/ Kara J. Nordstrom
                            --------------------------------------------
                             Name:  Kara J. Nordstrom
                                  --------------------------------------
                             Title: Vice President
                                   -------------------------------------


                         BANK OF AMERICA, N.A. (formerly Nations
                         Bank, N.A., formerly Boatmen's National Bank of
                         Oklahoma, formerly Bank IV Oklahoma, N.A.)


                         By:/s/ Michael S. Reeves
                            --------------------------------------------
                             Name:         Michael S. Reeves
                                   -------------------------------------
                             Title:    Senior Vice President
--                                  ------------------------------------


                         UMB OKLAHOMA BANK


                         By:/s/ Richard J. Lehrter
                            --------------------------------------------
                             Name:  Richard J. Lehrter
                                   -------------------------------------
                             Title: Executive Vice President
                                    ------------------------------------


                         SUMMIT BANK


                         By:/s/ Catherine E. Farrity
                            --------------------------------------------
                             Name:   Catherine E. Farrity
                                  --------------------------------------
                             Title: Vice President
                                   -------------------------------------


                         BANCFIRST


                         By:/s/ Brian K. Rewz
                            --------------------------------------------
                             Name:   Brian K. Rewz
                                  --------------------------------------
                             Title: Senior Vice President
                                   -------------------------------------
<PAGE>

Each Guarantor hereby (a) consents and agrees to this Amendment, (b) agrees that
its respective Guaranty shall continue to by the legal, valid and binding
obligation of such Guarantor enforceable against such Guarantor in accordance
with its terms, and (c) represents and warrants that each of the representations
and warranties set forth in this Amendment with regard to each such Guarantor
are true and correct in all respects.

                              GUARANTORS:
                              -----------

                              SONIC RESTAURANTS, INC.


                              By:/s/ W. Scott McLain
                                 -------------------------------------
                                     Name: W. Scott McLain
                                          ----------------------------
                                     Title:  Chief Financial Officer
                                           ---------------------------

                              SONIC INDUSTRIES  INC.


                              By:/s/ W. Scott McLain
                                 -------------------------------------
                                     Name:  W. Scott McLain
                                         -----------------------------
                                     Title:  Chief Financial Officer
                                          ----------------------------

                              AMERICA'S DRIVE-IN CORP.


                              By:/s/ W. Scott McLain
                                 -------------------------------------
                                     Name:   W. Scott McLain
                                          ----------------------------
                                     Title: Chief Financial Officer
                                           ---------------------------

                              AMERICA'S DRIVE-IN TRUST


                              By:/s/ W. Scott McLain
                                 -------------------------------------
                                     Name:   W. Scott McLain
                                          ----------------------------
                                     Title:  Chief Financial Officer
                                            --------------------------
<PAGE>

                              EACH OF THE PARTNERSHIPS SPECIFIED ON
                              ANNEX 1 HERETO, each an Oklahoma general
                              partnership


                              By:  Sonic Restaurants, Inc.,
                                             Managing General Partner of
                                             each of such partnerships


                                   By:/s/ W. Scott McLain
                                      ------------------------------------
                                      W. Scott McLain
                                           Chief Financial Officer
<PAGE>

                                    ANNEX 1

                                 Partnerships
                                 ------------

                 [List of Partnerships Intentionally Omitted]