<PAGE>   1
                                                                  Exhibit 10.6.1


                   AMENDMENT NO. 1 TO 364-DAY CREDIT AGREEMENT
                   -------------------------------------------


         This Amendment (this "AMENDMENT") is entered into as of September 29,
2000 by and among Nationwide Mutual Insurance Company ("NATIONWIDE MUTUAL"),
Nationwide Life Insurance Company ("NATIONWIDE LIFE"), Nationwide Financial
Services, Inc. ("NFS"; Nationwide Mutual, Nationwide Life and NFS are herein
referred to individually as a "BORROWER" and collectively as the "BORROWERS"),
Bank One, NA, individually and as agent (the "AGENT"), and the other financial
institutions signatory hereto.


                                    RECITALS
                                    --------

         A. The Borrowers, the Agent and the Lenders are party to that certain
364-Day Credit Agreement dated as of May 25, 2000 (the "CREDIT AGREEMENT").
Unless otherwise specified herein, capitalized terms used in this Amendment
shall have the meanings ascribed to them by the Credit Agreement.

         B. The Borrowers, the Agent and the undersigned Lenders wish to amend
the Credit Agreement on the terms and conditions set forth below.

         NOW, THEREFORE, in consideration of the mutual execution hereof and
other good and valuable consideration, the parties hereto agree as follows:

         1. AMENDMENT TO CREDIT AGREEMENT. Upon the "Effective Date" (as defined
below), the Credit Agreement shall be amended as follows:

            (a) Article I of the Credit Agreement is amended by inserting the
         following new definition in alphabetical order:

            "Five Year Credit Agreement" means that certain Five Year Credit
         Agreement, dated as of May 25, 2000, among the Borrowers, the financial
         institutions party thereto and Bank One, NA, as agent for such
         financial institutions.

            (b) Section 4.2(b) of the Credit Agreement is amended in its
         entirety to read as follows:

            (b) The representations and warranties of the applicable Borrower
         contained in ARTICLE V are true and correct as of such Borrower Date
         except to the extent any such representation or warranty is stated to
         relate solely to an earlier date, in which case such representation or
         warranty shall have been true and correct on and as of such earlier
         date; PROVIDED, HOWEVER, that Advances made to Nationwide Life under
         this Agreement for commercial paper back-up in an aggregate amount
         which, together with all Advances made to Nationwide Life for
         commercial paper back-up under the Five Year Credit Agreement, does not

<PAGE>   2


         exceed $300,000,000, will not be subject to the truthfulness of the
         representations and warranties set forth in SECTION 5.5.

         2. REPRESENTATIONS AND WARRANTIES OF THE BORROWERS. Each Borrower
represents and warrants that:

            (a) The execution, delivery and performance by such Borrower of this
         Amendment have been duly authorized by all necessary corporate
         proceedings and that this Amendment is a legal, valid and binding
         obligation of such Borrower enforceable against such Borrower in
         accordance with its terms, except as enforceability may be limited by
         bankruptcy, insolvency or similar law affecting creditors' rights
         generally and by general principles of equity;

            (b) Each of the representations and warranties of such Borrower
         contained in the Credit Agreement is true and correct on and as of the
         date hereof as if made on the date hereof except to the extent any such
         representation or warranty is stated to relate solely to an earlier
         date, in which case such representation or warranty was true and
         correct on and as of such earlier date;

            (c) After giving effect to this Amendment, no Default or Unmatured
         Default has occurred and is continuing with respect to such Borrower.

         3. EFFECTIVE DATE. SECTION 1 of this Amendment shall become effective
upon the execution and delivery hereof by the Borrowers, the Agent and the
Required Lenders (without respect to whether it has been executed and delivered
by all the Lenders).

         4. REFERENCE TO AND EFFECT UPON THE CREDIT AGREEMENT.

            (a) Except as specifically amended above, the Credit Agreement and
         the other Loan Documents shall remain in full force and effect and are
         hereby ratified and confirmed.

            (b) The execution, delivery and effectiveness of this Amendment
         shall not operate as a waiver of any right, power or remedy of the
         Agent or any Lender under the Credit Agreement or any Loan Document,
         nor constitute a waiver of any provision of the Credit Agreement or any
         Loan Document, except as specifically set forth herein. Upon the
         effectiveness of this Amendment, each reference in the Credit Agreement
         to "this Agreement", "hereunder", "hereof", "herein" or words of
         similar import shall mean and be a reference to the Credit Agreement as
         amended hereby.

         5. COSTS AND EXPENSES. Each Borrower hereby affirms its joint and
several obligation under Section 9.6 of the Credit Agreement to reimburse the
Agent for all reasonable costs, internal charges and out-of-pocket expenses paid
or incurred by the Agent in connection with the preparation, negotiation,
execution and delivery of this Amendment, including but not limited to the
attorneys' fees and time charges of attorneys for the Agent with respect
thereto.

         6. GOVERNING LAW. THIS AMENDMENT SHALL BE CONSTRUED IN ACCORDANCE WITH
THE INTERNAL LAWS (INCLUDING, WITHOUT LIMITATION,



                                      -2-
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735 ILCS SECTION 105/5-1 ET SEQ, BUT OTHERWISE WITHOUT REGARD TO THE CONFLICT OF
LAWS PROVISIONS) OF THE STATE OF ILLINOIS, BUT GIVING EFFECT TO FEDERAL LAWS
APPLICABLE TO NATIONAL BANKS.

         7. HEADINGS. Section headings in this Amendment are included herein for
convenience of reference only and shall not constitute a part of this Amendment
for any other purposes.

         8. COUNTERPARTS. This Amendment may be executed in any number of
counterparts, each of which when so executed shall be deemed an original but all
such counterparts shall constitute one and the same instrument.


                            [signature pages follow]



                                      -3-
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                  IN WITNESS WHEREOF, the parties have executed this Amendment
as of the date and year first above written.


                                  NATIONWIDE MUTUAL INSURANCE COMPANY


                                  By:
                                     ------------------------------------------
                                           Duane M. Campbell
                                  Title:   Vice President and Treasurer
                                        ---------------------------------------

                                  NATIONWIDE LIFE INSURANCE COMPANY


                                  By:
                                     ------------------------------------------
                                           Duane M. Campbell
                                  Title:   Vice President and Treasurer
                                        ---------------------------------------


                                  NATIONWIDE FINANCIAL SERVICES, INC.


                                  By:
                                     ------------------------------------------
                                           Duane M. Campbell
                                  Title:   Vice President and Treasurer
                                        ---------------------------------------


                                  BANK ONE, NA, as Agent and as a Lender


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


                                  THE CHASE MANHATTAN BANK, as a Lender


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







                                      S-1


                              [TO AMENDMENT NO. 1]

<PAGE>   5

                                  THE BANK OF NEW YORK, as a Lender


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


                                  CITICORP USA, INC., as a Lender


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


                                  MORGAN GUARANTY TRUST COMPANY OF NEW YORK,
                                  as a Lender


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


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


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


                                  FIRST UNION NATIONAL BANK, as a Lender


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


                                  MELLON BANK, N.A., as a Lender


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







                                      S-2


                              [TO AMENDMENT NO. 1]


<PAGE>   6

                                  FLEET NATIONAL BANK, as a Lender


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


                                  KEYBANK NATIONAL ASSOCIATION, as a Lender


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


                                  WELLS FARGO BANK, N.A., as a Lender


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


                                  FIRSTAR BANK, NA, as a Lender


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


                                  THE HUNTINGTON NATIONAL BANK, as a Lender


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


                                  NATIONAL CITY BANK, as a Lender


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








                                      S-3


                              [TO AMENDMENT NO. 1]
<PAGE>   7

                                  STATE STREET BANK AND TRUST COMPANY, as a
                                  Lender


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


                                  WACHOVIA BANK, NA, as a Lender


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


                                  FIFTH THIRD BANK, CENTRAL OHIO, as a Lender


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


                                  FIRST HAWAIIAN BANK, as a Lender


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







                                      S-4


                              [TO AMENDMENT NO. 1]