<PAGE>   1

                                                                    EXHIBIT 4.12

                                AMENDMENT NO. 10


         THIS AMENDMENT NO. 10, dated as of September 21, 2000 (the "AMENDMENT")
relating to the Credit Agreement referenced below, by and among NATIONAL MEDICAL
CARE, INC., a Delaware corporation, certain subsidiaries and affiliates party to
the Credit Agreement and identified on the signature pages hereto, and BANK OF
AMERICA, N.A., (formerly known as NationsBank, N.A), as Paying Agent for and on
behalf of the Lenders. Terms used but not otherwise defined shall have the
meanings provided in the Credit Agreement.

                               W I T N E S S E T H

         WHEREAS, a $2.5 billion credit facility has been extended to National
Medical Care, Inc. and certain subsidiaries and affiliates pursuant to the terms
of that Credit Agreement dated as of September 27, 1996 (as amended and
modified, the "CREDIT AGREEMENT") among National Medical Care, Inc., the other
Borrowers, Guarantors and the Lenders identified therein, and NationsBank, N.A.,
as Paying Agent.

         WHEREAS, on March 2, 2000, Holdings issued 8,974,359 of its non-voting
preference shares to a limited number of institutional and other accredited
investors in exchange for the contribution by those investors of all of the
membership interests in Franconia Acquisition LLC, a Delaware limited liability
company, ("FRANCONIA") dedicated to acquiring dialysis centers and related
businesses and having a cash balance of $350 million (the "FRANCONIA
TRANSACTION").

         WHEREAS, on July 26, 2000, Holdings issued 5,000,000 of its non-voting
preference shares in a global offering to fund acquisitions and capital
expenditures (together with the 750,000 shares issued in connection with the
overallotment option granted in connection herewith, the "2000 OFFERING");

         WHEREAS, the Company has requested the modification of certain
covenants and certain other changes to the Credit Agreement more fully set forth
herein;

         WHEREAS, the requested consents and modifications described herein
require the consent of the Required Lenders; and

         WHEREAS, the Required Lenders have consented to the requested
modifications on the terms and conditions set forth herein and have authorized
the Paying Agent to enter into this Amendment on their behalf to give effect to
this Amendment.

         NOW, THEREFORE, IN CONSIDERATION of these premises and other good and
valuable consideration, the receipt and sufficiency of which is hereby
acknowledged, the parties hereto agree as follows:

         1.       The Credit Agreement is amended and modified in the following
respects:

                  1.1      The following definitions in Section 1.1 are amended
         and modified as hereafter provided:

                           (i)      The definition of "Material Subsidiary" is
                  amended by inserting at the end of the first proviso
                  thereto the following:

                                    "nor any member of the Consolidated Group
                           party to any Acquired Subdebt that would be
                           prohibited from entering into the Guarantor Joinder


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                           Agreement or whose Parent would be prohibited from
                           pledging its stock pursuant to the Pledge Agreement,
                           in either case, pursuant to the provisions of such
                           Acquired Subdebt as in effect on the date that it
                           became a member of the Consolidated Group or pursuant
                           to the documents relating to the Subordinated Debt or
                           the Additional Subdebt."

                           (ii)     The definition of "Permitted Investments" is
                  amended in the following respects:

                                    (A) Clause (xiv) thereof is hereby amended
                           by deleting the reference to "$100,000,000" and
                           replacing it with a reference to "$200,000,000".

                                    (B) A new subsection (xxi) is hereby added
                           thereunder and subsection (xxi) thereof is renumbered
                           to be subsection (xxii), as follows:

                                            (xxi) Investments by Holdings or any
                                    Foreign Subsidiary of Holdings in preferred
                                    stock issued by WRG-NY or any Subsidiary of
                                    WRG-NY that is not a Credit Party in
                                    connection with the RIPSS/REPO structures
                                    described on ANNEX A to Amendment No. 10 to
                                    this Agreement or substantially similar
                                    transactions;

                           (iii) The definition of "Excluded Securitization
                  Transaction" is hereby amended by deleting the reference to
                  "$400,000,000" in the eighth line thereof and inserting a
                  reference to "$600,000,000" in its place.

                           (iv) The definition of "Permitted Receivables
                  Financing" is hereby amended by deleting the references to
                  "$15 million" and "$75 million" in clause (B) of the proviso
                  thereto and inserting references to "$20 million" and "$100
                  million", respectively, in place thereof.

                  1.2      Clause (b) of Section 7.11 is amended in its entirety
       to read as follows:

                           (b) If a Foreign Subsidiary which is a Material
                  Subsidiary has not either (i) become a Credit Party hereunder,
                  or (ii) had its capital stock or the capital stock of its
                  Parent pledged to secure the Obligations (or, if such Foreign
                  Subsidiary is a direct or indirect Subsidiary of WRG-NY or
                  other Domestic Credit Party, had 66% of its capital stock or
                  66% of the capital stock of its Parent pledged to secure the
                  Obligations hereunder) if such Foreign Subsidiary is a direct
                  or indirect Subsidiary of WRG-NY or other Domestic Credit
                  Party, then the Company will promptly notify the Paying Agent
                  thereof and WRG-NY will pledge, or if the Parent or such
                  Foreign Subsidiary is a Subsidiary of WRG-NY, will cause the
                  Parent to pledge, 66% of the capital stock of such Foreign
                  Subsidiary or 66% of the capital stock of such Parent and, in
                  connection therewith deliver a related pledge agreement and
                  such supporting resolutions, incumbency certificates,
                  corporate formation and organizational documentation and
                  opinions of counsel as the Paying Agent may reasonably
                  request.

                  1.3      Section 8.1 (Indebtedness) is amended in the
       following respects:

                           (i)      Clause (e) is hereby amended by deleting the
                  reference to "$400,000,000" in clause (i) of the proviso
                  thereto and inserting a reference to "$600,000,000" in its
                  place;

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                           (ii) Clause (m) is hereby amended by deleting the
                  reference to "$500 million" in the first line thereof (which
                  was increased to $650 million by consent of the Lenders in
                  February, 1998) and replacing it with a reference to "$950
                  million" and inserting the following additional proviso at the
                  end thereof:

                                    "PROVIDED, FURTHER that, up to $300 million
                           of such amount may be subordinated debt of any member
                           of the Consolidated Group and Guaranty Obligations
                           relating thereto, in either case existing at the time
                           such Person becomes a Subsidiary of Holdings so long
                           as such debt and obligations were not incurred by
                           such Person in connection with, or in anticipation or
                           contemplation of the acquisition, merger or
                           consolidation pursuant to which such Person became a
                           member of the Consolidated Group ("ACQUIRED SUBDEBT")
                           and shall not be subject to the requirements of
                           clause (ii) of the first proviso hereto (and clauses
                           (i) and (iii) thereof shall not be applicable to such
                           Acquired Subdebt except that such Acquired Subdebt
                           may not be guaranteed by Holdings or any other
                           Subsidiary of Holdings not created in contemplation
                           of the acquisition without the prior written consent
                           of the Required Lenders)."

                  1.4 Section 8.4(c) is hereby amended by renumbering clause
         (vi) thereof as clause (vii) and inserting a new clause (vi) thereto as
         follows:

                           (vi) the transfer of an interest in any accounts
                  receivable from any Subsidiary of WRG-NY that is a special
                  purpose accounts receivable financing vehicle to WRG-NY or any
                  other Subsidiary of WRG-NY in connection with the RIPSS/REPO
                  structure described on ANNEX A to Amendment No. 10 to this
                  Agreement or any other substantially similar transaction.

                  1.5 Clause (b) of Section 8.9 is hereby amended by inserting
         the words "(other than as required by the terms of the Acquired
         Subdebt, within six months of the related acquisition or on other terms
         reasonably acceptable to the Paying Agent and the Required Lenders)"
         immediately before the words "other than regularly scheduled interest
         payments" in line 4 thereof.

                  1.6 Section 8.12 is hereby amended by deleting the word "or"
         before the words "requiring the grant" in line 7 thereof and replacing
         it with a comma "," and inserting the words "or existing pursuant to
         the provisions of any Acquired Subdebt as in effect on the date that
         the Person obligated on such debt became a member of the Consolidated
         Group" at the end thereof.

         2.       ACKNOWLEDGMENTS AND CONSENTS WITH RESPECT TO THE RIPSS/REPO
SECURITIES.

                  2.1 The obligations of any member of the Consolidated Group
         under any Option Agreement entered into with any other member of the
         Consolidated Group to buy or sell equity interests in any Subsidiary of
         WRG-NY or any Subsidiary of WRG-NY shall not be considered "Guaranty
         Obligations" as such term is used and defined in the Credit Agreement.

                  2.2 Any Material Subsidiary that is a special purpose entity
         created for the purpose of entering into any Option Agreement for the
         purchase or sale of any equity interests described in the foregoing
         clause 2.1 shall not be required to enter into the Guarantor Joinder
         Agreement, nor shall its Parent be required to pledge its stock under
         the Pledge Agreement.

                  2.3 For the purposes of clarification, it is hereby understood
         and agreed that the segregation of a pool of assets for the purposes of
         making an election to treat those assets as a "Financial Asset
         Securitization Investment Trust" or "FASIT" under the Internal Revenue
         Code

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<PAGE>   4


         or any successor statute or the transfer of any ownership interest
         therein as contemplated by the RIPSS/REPO structure described on ANNEX
         A to Amendment No. 10 to the Credit Agreement or substantially similar
         transactions shall not constitute a "Lien" as such term is used and
         defined in the Credit Agreement.

         3.       ACKNOWLEDGMENTS AND CONSENTS WITH RESPECT TO THE FRANCONIA
TRANSACTION AND PUBLIC OFFERING.

                  3.1 It is hereby understood and agreed that (i) no portion of
         the Net Proceeds of the Franconia Transaction shall be required to be
         applied to the prepayment of the Loans and the reduction of the
         Commitments as provided in Section 3.3(b), even if the acquisitions to
         which the cash held by Franconia is applied do not occur within the
         time periods contemplated by Section 8.4(d) and (ii) no portion of the
         Net Proceeds of the 2000 Offering shall be required to be applied to
         the prepayment of the Loans and the reduction of the Commitments as
         provided in Section 3.3(b), and the provisions of Section 3.3(b) of the
         Credit Agreement are hereby waived with respect to the Net Proceeds of
         the Franconia Transaction and the 2000 Offering; PROVIDED that the cash
         held by Franconia on the date of the consummation of the Franconia
         Transaction and the Net Proceeds of the 2000 Offering shall continue to
         be treated as Net Proceeds of an Equity Transaction for all other
         purposes under the Credit Agreement.

                  3.2 It is hereby agreed that neither Franconia Acquisition LLC
         nor any Domestic Subsidiary of Franconia Acquisition LLC shall be
         considered to be a Material Subsidiary and Holdings shall not be
         required to cause the stock of such entities to be pledged to secure
         the Obligations nor shall such entities be required to execute the
         Guarantor Joinder Agreement, and the provisions of Section 7.11 are
         hereby waived to such extent.

         4. The Lenders hereby waive compliance with the provisions of the
Credit Agreement as in effect before the execution and delivery of this
Amendment No. 10 to the extent, and only to the extent, that any transaction or
action of any member of the Consolidated Group would have been permitted by the
provisions of the Credit Agreement as amended hereby.

         5. The effectiveness of this Amendment is subject to receipt by
the Paying Agent of the following:

                  (i)      copies of this Amendment executed by the Company and
         the other members of the Consolidated Group identified on the signature
         pages hereto,

                  (ii)     the consent of the Required Lenders, and

                  (iii) an Amendment Fee equal to five basis points (0.05%) of
         the aggregate amount of the Commitments held by the Lenders consenting
         to this Amendment for the ratable benefit of such consenting Lenders.

         6. Except as modified hereby, all of the terms and provisions of
the Credit Agreement (and Exhibits and Schedules) remain in full force and
effect.

         7. The Credit Parties hereby affirm (i) the representations and
warranties set out in Section 6 of the Credit Agreement are true and correct as
of the date hereof (except those which expressly relate to an earlier period)
and (ii) no Default or Event of Default presently exists.

         8. The Company agrees to pay all reasonable costs and expenses of the
Paying Agent in connection with the preparation, execution and delivery of this
Amendment, including without limitation the reasonable fees and expenses of
Moore & Van Allen, PLLC.

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<PAGE>   5

         9. This Amendment may be executed in any number of counterparts, each
of which when so executed and delivered shall be deemed an original and its
shall not be necessary in making proof of this Amendment to produce or account
for more than one such counterpart.

        10. This Amendment, and the Credit Agreement as amended hereby, shall
be governed by and construed and interpreted in accordance with the laws of the
State of New York.


                  [Remainder of Page Intentionally Left Blank]


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         IN WITNESS WHEREOF, each of the parties hereto has caused a counterpart
of this Amendment to be duly executed and delivered as of the date first above
written.


BORROWERS:                    NATIONAL MEDICAL CARE, INC.,
                              a Delaware corporation

                              By:    /s/ Ramon Yi
                                 -----------------------------------------------
                              Name:  Ramon Yi
                              Title: Treasurer

                              FRESENIUS MEDICAL CARE AG

                              By:    /s/ Dr. Emanuele Gotti
                                 -----------------------------------------------
                              Name:  Dr. Emanuele Gotti
                              Title: Member of the Board of Management

                              By:     /s/ Dr. Werner Brandt
                                 -----------------------------------------------
                              Name:   Dr. Werner Brandt
                              Title:  Member of the Board of Management

                              NMC DO BRASIL LTDA.,
                              a Brazil corporation

                              By:     /s/ Joao Pedrinelli
                                 -----------------------------------------------
                              Name:   Joao Pedrinelli
                              Title:  Member of the Board of Management

                              NATIONAL MEDICAL CARE OF SPAIN, S.A.,
                              a Spanish corporation

                              By:  /s/ Dr. Emanuele Gotti /s/ Dr. Andrea Stopper
                                 -----------------------------------------------
                              Name:  Dr. Emanuele Gotti       Dr. Andrea Stopper
                              Title: Board Member             Board Member

                              NATIONAL MEDICAL CARE OF TAIWAN, INC.,
                              a Delaware corporation

                              By:    /s/ Roberto Fuste        /s/ Betty Na
                                 -----------------------------------------------
                              Name:  Roberto Fuste                Betty Na
                              Title: Members of the Board of Directors

                              NMC CENTRO MEDICO NACIONAL, LDA.,
                              a Portuguese corporation

                              By:    /s/ Ricardo Da Silva    /s/ John Allen
                                 -----------------------------------------------
                              Name:  Ricardo Da Silva            John Allen
                              Title: Board Members

                              FRESENIUS MEDICAL CARE ARGENTINA, S.A.,
                              as successor by merger to

                                       6
<PAGE>   7

                              NMC DE ARGENTINA, S.A.,
                              an Argentine corporation

                              By:    /s/ Dr. Guido Yagupsky  /s/ Horst Radthe
                                 -----------------------------------------------
                              Name:  Dr. Guido Yagupsky          Horst Radthe
                              Title: Board Members

                              FRESENIUS USA, INC.,
                              a Massachusetts corporation

                              By:    /s/ Ramon Yi
                                 -----------------------------------------------
                              Name:  Ramon Yi
                              Title: Treasurer

                              FRESENIUS MEDICAL CARE DEUTSCHLAND GmbH,
                              a German corporation

                              By:     /s/ Dr. Emanuele Gotti
                                 -----------------------------------------------
                              Name:   Dr. Emanuele Gotti
                              Title:  Board Member

                              By:    /s/ Dr. Werner Brandt
                                 -----------------------------------------------
                              Name:  Dr. Werner Brandt
                              Title: Board Member

                              FRESENIUS MEDICAL CARE GROUPE FRANCE
                              (formerly known as Fresenius Groupe France S.A.),
                              a French corporation

                              By:    /s/ Udo Werle      /s/ Dr. Emanuele Gotti
                                 -----------------------------------------------
                              Name:  Udo Werle              Dr. Emanuele Gotti
                              Title: Board Members

                              FRESENIUS MEDICAL CARE HOLDING, S.p.A.,
                              an Italian corporation

                              By:    /s/ Dr. Emanuele Gotti  /s/ Andrea Stopper
                                 -----------------------------------------------
                              Name:  Dr. Emanuele Gotti          Andrea Stopper
                              Title: Board Members

                              FRESENIUS MEDICAL CARE ESPANA S.A.,
                              a Spanish corporation

                              By:     /s/ Ricardo Davel     /s/ Manuel Gluete
                                 -----------------------------------------------
                              Name:   Ricardo Davel             Manuel Gluete
                              Title:  Board Members

                              FRESENIUS MEDICAL CARE MAGYAROSZA KfG,
                              a Hungarian corporation


                                       7
<PAGE>   8


                             By:    /s/ Norman Erhard
                                ------------------------------------------------
                             Name:  Norman Erhard
                             Title: Board Member

                             BIO-MEDICAL APPLICATIONS OF ALABAMA, INC.

                             By:    /s/ Ramon Yi
                                ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer

                             BIO-MEDICAL APPLICATIONS OF FLORIDA, INC.

                             By:    /s/ Ramon Yi
                                 -----------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer

                             BIO-MEDICAL APPLICATIONS OF GEORGIA, INC.

                             By:    /s/ Ramon Yi
                                ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer

                             BIO-MEDICAL APPLICATIONS OF INDIANA, INC.

                             By:    /s/ Ramon Yi
                                 -----------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer

                             BIO-MEDICAL APPLICATIONS OF KENTUCKY, INC.

                             By:    /s/ Ramon Yi
                                ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer

                             BIO-MEDICAL APPLICATIONS OF LOUISIANA, INC.

                             By:    /s/ Ramon Yi
                                ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer


                                       8
<PAGE>   9


                             BIO-MEDICAL APPLICATIONS OF MARYLAND, INC.

                             By:    /s/ Ramon Yi
                                ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer

                             BIO-MEDICAL APPLICATIONS OF MASSACHUSETTS, INC.

                             By:    /s/ Ramon Yi
                                ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer

                             BIO-MEDICAL APPLICATIONS OF NORTH CAROLINA, INC.

                             By:    /s/ Ramon Yi
                                ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer

                             BIO-MEDICAL APPLICATIONS OF OHIO, INC.

                             By:    /s/ Ramon Yi
                               ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer

                             BIO-MEDICAL APPLICATIONS OF PENNSYLVANIA, INC.

                             By:    /s/ Ramon Yi
                                ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer

                             BIO-MEDICAL APPLICATIONS OF SOUTH CAROLINA, INC.

                             By:    /s/ Ramon Yi
                                ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer

                             BIO-MEDICAL APPLICATIONS OF TEXAS, INC.

                             By:    /s/ Ramon Yi
                                ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer


                                       9
<PAGE>   10


                             BIO-MEDICAL APPLICATIONS OF VIRGINIA, INC.

                             By:    /s/ Ramon Yi
                                ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer

                             LIFECHEM, INC.,
                             a Delaware corporation

                             By:    /s/ Ramon Yi
                                ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer


GUARANTORS:                  FRESENIUS MEDICAL CARE HOLDINGS, INC.,
                             a New York corporation formerly known as WRG-NY

                             By:    /s/ Ramon Yi
                                ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer

                             NATIONAL MEDICAL CARE, INC.,
                             a Delaware corporation

                             By:    /s/ Ramon Yi
                                ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer

                             BIO-MEDICAL APPLICATIONS MANAGEMENT CO.,
                             INC., a Delaware corporation

                             By:    /s/ Ramon Yi
                                ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer

                             FRESENIUS MEDICAL CARE AG,
                             a German corporation

                             By:    /s/ Dr. Emanuele Gotti
                                ------------------------------------------------
                             Name:  Dr. Emanuele Gotti
                             Title: Board Member

                             By:     /s/ Dr. Werner Brandt
                                ------------------------------------------------
                             Name:   Dr. Werner Brandt
                             Title:  Board Member


                                       10
<PAGE>   11


                             FRESENIUS USA, INC.,
                             a Massachusetts corporation

                             By:    /s/ Ramon Yi
                                ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer

                             FRESENIUS MEDICAL CARE DEUTSCHLAND
                             GmbH, a German corporation

                             By:     /s/ Dr. Emanuele Gotti
                                ------------------------------------------------
                             Name:   Dr. Emanuele Gotti
                             Title:  Board Member

                             By:     /s/ Dr. Werner Brandt
                                ------------------------------------------------
                             Name:   Dr. Werner Brandt
                             Title:  Board Member

                             FRESENIUS MEDICAL CARE GROUPE FRANCE,
                             a French corporation
                             (formerly known as Fresenius Groupe
                             France S.A.)

                             By:     /s/ Dr. Udo Werle  /s/ Dr. Emanuele Gotti
                                ------------------------------------------------
                             Name:   Dr. Udo Werle          Dr. Emanuele Gotti
                             Title:  Board Members

                             FRESENIUS SECURITIES, INC.,
                             a California corporation

                             By:    /s/ Ramon Yi
                                ------------------------------------------------
                             Name:  Ramon Yi
                             Title: Treasurer

                             NEOMEDICA, INC.,
                             a Delaware corporation

                             By:    /s/ Doug Kott
                                ------------------------------------------------
                             Name:  Doug Kott
                             Title: Secretary

                             FMC FINANCE S.A.,
                             a Luxembourg corporation

                             By:    /s/ John Allen
                                ------------------------------------------------
                             Name:  John Allen
                             Title: Board Member


                                       11
<PAGE>   12


                             FMC TRUST FINANCE S.a.r.l. LUXEMBOURG,
                             a Luxembourg corporation

                             By:    /s/ Andrea Stopper
                                ------------------------------------------------
                             Name:  Andrea Stopper
                             Title: Board Member


PAYING AGENT:                BANK OF AMERICA, N.A. (formerly known as
                             NationsBank, N.A.), as Paying Agent for and
                             on behalf of the Lenders

                             By:     /s/ Ashley M. Crabtree
                                ------------------------------------------------
                             Name:   Ashley M. Crabtree
                             Title:  Senior Vice President


                                       12

<PAGE>   13


                           CONSENT TO AMENDMENT NO. 10


Bank of America, N.A. (formerly known as NationsBank, N.A.),
  as Paying Agent
101 N. Tryon Street, 15th Floor
NC1-001-15-04
Charlotte, North Carolina 28255
Attn:  James D. Young, Agency Services

     Re:   Credit Agreement dated as of September 27, 1996 (as amended and
           modified, the "CREDIT AGREEMENT") among National Medical Care, Inc.,
           the other Borrowers, Guarantors and Lenders identified therein and
           NationsBank, N.A. (now known as Bank of America, N.A.), as Paying
           Agent. Terms used but not otherwise defined shall have the meanings
           provided in the Credit Agreement.

           Amendment No. 10 dated September 21, 2000 (the "SUBJECT AMENDMENT")
           relating to the Credit Agreement

Ladies and Gentlemen:

         This should serve to confirm our receipt of, and consent to, the
Subject Amendment. We hereby authorize and direct you, as Paying Agent for the
Lenders, to enter into the Subject Amendment on our behalf in accordance with
the terms of the Credit Agreement upon your receipt of such consent and
direction from the Required Lenders, and agree that Company and the other Credit
Parties may rely on such authorization.

                                         Sincerely,



                                         ---------------------------------------
                                                  [Name of Lender]

                                         By:____________________________________

                                         Name:

                                         Title:


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