<PAGE>   1

                                                                   EXHIBIT 10.22

                               THIRD AMENDMENT TO
                                CREDIT AGREEMENT


        THIS THIRD AMENDMENT TO CREDIT AGREEMENT is made and dated as of
December 8, 1999 (the "AMENDMENT") among PACIFICARE HEALTH SYSTEMS, INC., a
Delaware corporation (the "COMPANY"), the Banks party to the Credit Agreement
referred to below, and BANK OF AMERICA, N.A., a national banking association, as
Agent (the "AGENT"), and amends that certain Credit Agreement dated as of
October 31, 1996, as amended by that certain First Amendment to Credit Agreement
dated as of August 15, 1997 and that certain Second Amendment to Credit
Agreement dated as of December 31, 1997 (as so amended, the "CREDIT AGREEMENT").


                                    RECITALS

        WHEREAS, the Company has requested the Agent and the Banks to amend
certain provisions of the Credit Agreement, and the Agent and the Banks are
willing to do so, on the terms and conditions specified herein;

        NOW, THEREFORE, for good and valuable consideration, the receipt and
adequacy of which are hereby acknowledged, the parties hereby agree as follows:

        1. Terms. All terms used herein shall have the same meanings as in the
Credit Agreement unless otherwise defined herein.

        2. Amendment. The Credit Agreement is hereby amended as follows:

               2.1 Amendments to Section 1.1.

                      (a) The definitions of the terms "Applicable Level" and
        "Applicable Margin" in Section 1.1 of the Credit Agreement are hereby
        amended and restated to read in their entirety as follows:

               "Applicable Level" means one of the levels set forth below
        determined by the Senior Unsecured Debt Rating as follows:

                      "Level 1" means any period during which the Senior
               Unsecured Debt Rating is better than or equal to (i) BBB+ by S&P
               and/or (ii) Baa1 by Moody's.



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<PAGE>   2

                      "Level 2" means any period (other than a Level 1 Period)
               during which the Senior Unsecured Debt Rating is better than or
               equal to (i) BBB by S&P and/or (ii) Baa2 by Moody's.

                      "Level 3" means any period (other than a Level 1 Period or
               Level 2 Period) during which the Senior Unsecured Debt Rating is
               better than or equal to (i) BBB- by S&P and/or (ii) Baa3 by
               Moody's.

                      "Level 4" means any period (other than a Level 1 Period,
               Level 2 Period or Level 3 Period) during which the Senior
               Unsecured Debt Rating is better than or equal to (i) BB+ by S&P
               and/or (ii) Ba1 by Moody's.

                      "Level 5" means any period (other than a Level 1 Period,
               Level 2 Period, Level 3 Period or Level 4 Period) during which
               the Senior Unsecured Debt Rating is better than or equal to (i)
               BB by S&P and/or (ii) Ba2 by Moody's.

                      "Level 6" means any period other than a Level 1 Period,
               Level 2 Period, Level 3 Period, Level 4 Period or Level 5 Period.

               For purposes of the foregoing, (a) if the Senior Unsecured Debt
        Ratings fall within different Levels, the Applicable Level shall be
        based upon the higher (numerically lower) of the available Levels unless
        such Levels are more than one Level apart, in which case the Applicable
        Level shall be one Level higher than the lower Level; (b) if only one
        Senior Unsecured Debt Rating exists, the Applicable Level shall be based
        upon the Level in which such rating falls; and (c) if no Senior
        Unsecured Debt Rating shall be available from at least one of S&P or
        Moody's, the Applicable Level shall be Level 6.

               "Applicable Margin" means, in the case of Facility Fees, Base
        Rate Committed Loans or LIBOR Committed Loans, a rate per annum
        determined by reference to the Applicable Level as follows:

<TABLE>
<CAPTION>
                          Applicable Base     Applicable LIBOR
  Applicable Level          Rate Margin         Rate Margin        Facility Fee
---------------------     ---------------     ----------------     ------------
<S>                       <C>                 <C>                  <C>
Level 1                          0.0%               0.625%            0.175%
Level 2                          0.0%               0.800%            0.200%
Level 3                          0.0%               1.000%            0.250%
Level 4                        0.125%               1.125%            0.375%
Level 5                        0.500%               1.500%            0.500%
Level 6                        1.000%               2.000%            0.750%
</TABLE>



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<PAGE>   3

               Changes in the Applicable Margin shall take effect (i) in the
        case of the Applicable LIBOR Rate Margin for LIBOR Loans, at the
        beginning of the following Interest Period and (ii) otherwise, as of the
        date of public announcement by S&P or Moody's, as applicable.

                      (b) There shall be added to Section 1.1 of the Credit
        Agreement, in appropriate alphabetical sequence, a new definition
        reading in its entirety as follows:

               "Investment Grade Rating Date" means the date upon which the
        Company has received Senior Unsecured Debt Ratings of BBB- or better by
        S&P and Baa3 or better by Moody's.

               2.2 Amendment to Section 2.9. Section 2.9 of the Credit Agreement
is hereby amended by deleting the date "January 1, 2000" and replacing it with
"the effective date of the Third Amendment dated as of December 8, 1999 to this
Agreement".

               2.3 Amendments to Section 7.1. Clauses (a) and (b) of Section 7.1
of the Credit Agreement are hereby amended and restated in their entirety to
read as follows:

                      "(a) prior to the Investment Grade Rating Date, the
        Company or any of its Subsidiaries may make an Approved Acquisition or
        may merge or consolidate with or into another Person in an Approved
        Merger so long as the aggregate value of the cash, stock or other
        consideration (including Indebtedness assumed by the Company or its
        Subsidiaries in connection therewith) for any such Approved Acquisition
        or Approved Merger (exclusive of the acquisition of Harris Methodist
        Hospital) does not exceed $150,000,000 or such greater amount as may be
        approved in writing by the Majority Banks;

                      (b) after the Investment Grade Rating Date, the Company or
        any of its Subsidiaries may make an Approved Acquisition or may merge or
        consolidate with or into another Person in an Approved Merger; provided
        that in the case of an Approved Acquisition or an Approved Merger
        wherein the aggregate value of the cash, stock or other consideration
        (including Indebtedness assumed by the Company or its Subsidiaries in
        connection therewith) exceeds or is expected to exceed $150,000,000 (i)
        the Company shall deliver to the Agent: (A) a written description of
        such Acquisition, merger or consolidation; and (B) if requested by the
        Agent, copies of all agreements and Governmental Approvals relating to
        such Acquisition, merger or consolidation and evidence, that such
        Acquisition, merger or consolidation is an Approved Acquisition or an
        Approved Merger; and (ii) the Company shall calculate and deliver to the
        Agent prior to the consummation of such Acquisition or Approved Merger,
        the covenants set forth in Section 7.10 showing compliance therewith on
        a pro



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<PAGE>   4

        forma basis as though such Acquisition or Approved Merger had been
        consummated on the first day of the fourth fiscal quarter immediately
        prior to the date of determination;"

               2.4 Amendment to Section 7.7. Section 7.7 of the Credit Agreement
is hereby amended and restated in its entirety to read as follows:

               "7.7 Restricted Payments. The Company shall not, and shall not
        suffer or permit any Subsidiary to, declare or make any dividend payment
        or other distribution of assets, properties, cash, rights, obligations
        or securities on account of any shares of any class of its capital
        stock, or purchase, redeem or otherwise acquire for value any shares of
        its capital stock or any warrants, rights or options to acquire such
        shares, now or hereafter outstanding; except that

                      (a) the Company may declare and make dividend payments or
        other distributions payable solely in its common stock;

                      (b) any Subsidiary may declare, pay dividends or make
        other distributions to its shareholders so long as the same is done on a
        nondiscriminatory basis;

                      (c) the Company may purchase, redeem or otherwise acquire
        shares of its common stock or options to acquire any such shares with
        the proceeds received from the substantially concurrent issue of new
        shares of its common stock; and

                      (d) from and after December 1, 1997, the Company may
        expend up to $1,000,000,000 in connection with the repurchase of its
        capital stock, provided, that, immediately after giving effect to such
        proposed action, there exists no Default or Event of Default; and
        provided, further, that the aggregate amount of such repurchases in 2001
        shall not exceed 909,500 shares."

        3. Representations and Warranties. The Company represents and warrants
to the Agent and the Banks that, on and as of the date hereof, and after giving
effect to this Amendment:

               3.1 Authorization. The execution, delivery and performance by the
Company of this Amendment has been duly authorized by all necessary corporate
action, and this Amendment has been duly executed and delivered by the Company.

               3.2 Binding Obligation. This Amendment constitutes the legal,
valid and binding obligations of the Company, enforceable against it in
accordance with its terms, except as enforceability may be limited by applicable
bankruptcy, insolvency, or similar laws



                                       4
<PAGE>   5

affecting the enforcement of creditors' rights generally or by equitable
principles relating to enforceability.

               3.3 No Legal Obstacle to Amendment. The execution, delivery and
performance by the Company of this Amendment has been duly authorized by all
necessary corporate action, and does not and will not:

                      (a) contravene the terms of any of the Company's
        Organization Documents;

                      (b) conflict with in any material respect or result in any
        material breach or contravention of, or the creation of any Lien under,
        any document evidencing any material Contractual Obligation to which the
        Company is a party or any order, injunction, writ or decree of any
        Governmental Authority to which the Company or its property is subject;
        or

                      (c) violate any material Requirement of Law.

               3.4 Governmental Authorization. No approval, consent, exemption,
authorization, or other action by, or notice to, or filing with, any
Governmental Authority is necessary or required in connection with the
execution, delivery or performance by, or enforcement against the Company of
this Amendment other than that which has been obtained.

               3.5 Incorporation of Certain Representations. The representations
and warranties of the Company set forth in Article V of the Credit Agreement are
true and correct in all respects on and as of the date hereof as though made on
and as of the date hereof, except as to such representations made as of an
earlier specified date.

               3.6 Default. No Default or Event of Default under the Credit
Agreement has occurred and is continuing.

        4. Conditions, Effectiveness. The effectiveness of this Amendment shall
be subject to the compliance by the Company with its agreements herein
contained, and to the delivery of the following to Agent in form and substance
satisfactory to Agent:

               4.1 Authorized Signatories. A certificate, signed by the
Secretary or an Assistant Secretary of the Company and dated the date of this
Amendment, as to the incumbency of the person or persons authorized to execute
and deliver this Amendment and any instrument or agreement required hereunder on
behalf of the Company.

               4.2 Authorizing Resolutions. A certificate, signed by the
Secretary or an Assistant Secretary of the Company and dated



                                       5
<PAGE>   6

the date of the Amendment, as to the resolutions of the Company's board of
directors authorizing the transactions contemplated by this Amendment.

               4.3 Amendment Fees. Payment to the Agent, for the pro rata
benefit of each Bank approving this Amendment on or before 3:00 p.m., Pacific
standard time, on December 17, 1999, of an amendment fee in an amount equal to
 .375% of the aggregate amount of the Commitments (after giving effect to the
reduction thereof pursuant to Section 2.2 hereof) held by the Banks that have
executed and delivered this Amendment by such time; payment to the Agent, for
the pro rata benefit of each Bank approving this Amendment from and after 3:00
p.m., Pacific standard time, on December 17, 1999 but on or before 3:00 p.m.,
Pacific standard time, on December 23, 1999, of an amendment fee in an amount
equal to .25% of the aggregate amount of the Commitments (after giving effect to
the reduction thereof pursuant to Section 2.2 hereof) held by the Banks that
have executed and delivered this Amendment during such period; and payment of
all other fees and expenses of the Agent in connection with this Amendment
(including, without limitation, the fees and expenses of the counsel to the
Agent).

               4.4 Guarantor Affirmation. An acknowledgment and reaffirmation
letter in the form of Exhibit A hereto, duly executed by the Guarantor.

               4.5 Other Evidence. Such other evidence with respect to the
Company or any other person as the Agent or any Bank may reasonably request to
establish the consummation of the transactions contemplated hereby, the taking
of all corporate action in connection with this Amendment and the Agreement and
the compliance with the conditions set forth herein.

        5. Miscellaneous.

               5.1 Effectiveness of the Credit Agreement and the Notes. Except
as hereby expressly amended, the Credit Agreement and the Notes shall each
remain in full force and effect, and are hereby ratified and confirmed in all
respects on and as of the date hereof.

               5.2 Waivers. This Amendment is limited solely to the matters
expressly set forth herein and is specific in time and in intent and does not
constitute, nor should it be construed as, a waiver or amendment of any other
term or condition, right, power or privilege under the Credit Agreement or under
any agreement, contract, indenture, document or instrument mentioned therein;
nor does it preclude or prejudice any rights of the Agent or the Banks
thereunder, or any exercise thereof or the exercise of any other right, power or
privilege, nor shall it require the Banks



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<PAGE>   7

to agree to an amendment, waiver or consent for a similar transaction or on a
future occasion, nor shall any future waiver of any right, power, privilege or
default hereunder, or under any agreement, contract, indenture, document or
instrument mentioned in the Credit Agreement, constitute a waiver of any other
right, power, privilege or default of the same or of any other term or
provision.

               5.3 Counterparts. This Amendment may be executed in any number of
counterparts, and all of such counterparts taken together shall be deemed to
constitute one and the same instrument.


               5.4 Governing Law. This Amendment shall be governed by and
construed in accordance with the laws of the State of California.



                                       7
<PAGE>   8


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



                                        PACIFICARE HEALTH SYSTEMS, INC.


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        BANK OF AMERICA, N.A., as Agent


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


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


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        THE CHASE MANHATTAN BANK


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        CITICORP USA, INC.


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        THE BANK OF NEW YORK


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________



                                       8
<PAGE>   9

                                        THE BANK OF NOVA SCOTIA


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        BANQUE NATIONALE DE PARIS


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        THE DAI-ICHI KANGYO BANK, LTD.,
                                        CHICAGO BRANCH


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        THE INDUSTRIAL BANK OF JAPAN, LIMITED,
                                        LOS ANGELES AGENCY


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        COOPERATIEVE CENTRALE RAIFFEISEN-
                                        BOERENLEENBANK B.A., "RABOBANK
                                        NEDERLAND" NEW YORK BRANCH


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        By: ____________________________________
                                        Name: __________________________________



                                       9
<PAGE>   10

                                        Title: _________________________________


                                        SANWA BANK CALIFORNIA


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        THE SUMITOMO BANK, LIMITED,
                                        LOS ANGELES BRANCH


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        WELLS FARGO BANK, N.A.


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        BANCA COMMERCIALE ITALIANA
                                        LOS ANGELES FOREIGN BRANCH


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________



                                       10
<PAGE>   11

                                        BANQUE PARIBAS


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        CIBC INC.


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        COMMERZBANK AKTIENGESELLSCHAFT,
                                        LOS ANGELES BRANCH


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        CREDIT LYONNAIS NEW YORK BRANCH


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        CREDIT SUISSE FIRST BOSTON


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________



                                       11
<PAGE>   12

                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        BANK ONE, NA


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        THE FUJI BANK, LIMITED


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        MELLON BANK, N.A.


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        PNC BANK, N.A.


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________



                                       12
<PAGE>   13

                                        THE SAKURA BANK, LTD.,
                                        LOS ANGELES AGENCY


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        SOCIETE GENERALE


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        THE TOKAI BANK, LIMITED,
                                        LOS ANGELES AGENCY


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        UNION BANK OF CALIFORNIA, N.A.


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________



                                       13
<PAGE>   14

                                        SUNTRUST BANK, CENTRAL FLORIDA, N.A.


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________



                                       14
<PAGE>   15

                                        THE SANWA BANK, LIMITED,
                                        LOS ANGELES BRANCH


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        MORGAN GUARANTY TRUST COMPANY OF
                                        NEW YORK


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________


                                        BANK HAPOALIM B.M.


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________



                                        AIB INTERNATIONAL FINANCE


                                        By: ____________________________________
                                        Name: __________________________________
                                        Title: _________________________________



                                       15
<PAGE>   16

                                                                       EXHIBIT A
                                                              TO THIRD AMENDMENT
                                                             TO CREDIT AGREEMENT



                                December 8, 1999



PacifiCare Health Plan Administrators, Inc.,
successor by merger to
FHP International Corporation and
PacifiCare Operations, Inc.
5995 Plaza Drive
Cypress, California 90630

Attention: Chief Financial Officer

        Re:    Credit Agreement dated as of October 31, 1996

Ladies and Gentlemen:

        Please refer to (i) the Credit Agreement dated as of October 31, 1996,
as amended by that certain First Amendment to Credit Agreement dated as of
August 15, 1997 and that certain Second Amendment to Credit Agreement dated as
of December 31, 1997, by and among PacifiCare Health Systems, Inc., as the
borrower, the commercial lending institutions party thereto (the "Banks"),
various co-agents, various managing agents and Bank of America, N.A., as agent
(in such capacity, the "Agent") and (ii) the Guaranty dated as of October 31,
1996 from PacifiCare Operations, Inc. and the Guaranty dated as of February 14,
1997 of FHP International Corporation (each, a "Guaranty" and collectively, the
"Guaranties"). We understand that you are the successor by merger to PacifiCare
Operations, Inc. and FHP International Corporation. Pursuant to an amendment of
even date herewith, certain terms of the Credit Agreement were amended. We
hereby request that you (i) acknowledge and reaffirm all of your obligations and
undertakings under the Guaranties and (ii) acknowledge and agree that the
Guaranties are and shall remain in full force and effect in accordance with the
terms thereof.



<PAGE>   17

PacifiCare Health Plan Administrators, Inc.
December 8, 1999
Page 2



        Please indicate your agreement to the foregoing by signing in the space
provided below, and returning the executed copy to the undersigned.



                                            Very truly yours,

                                            BANK OF AMERICA, N.A., as Agent



                                            By: ________________________________
                                                Title: _________________________

Acknowledged and Agreed to:

PacifiCare Health Plan Administrators, Inc.,
successor by merger to
PACIFICARE OPERATIONS, INC. and
FHP INTERNATIONAL CORPORATION


By: ________________________________________________
    Its: ___________________________________________