EXHIBIT 10.5

                            VASTAR RESOURCES, INC.
                              FIRST AMENDMENT TO
                            EXECUTIVE MEDICAL PLAN

          Pursuant to the power of amendment reserved therein, Vastar Resources,
Inc. Executive Medical Plan (the "Plan") is hereby amended as follows:

          1.  Paragraph 1(c) is amended, effective as of the Effective Time (as
defined in Annex A to the Plan as amended hereby), to read as follows:

              "(c)  "Benefit Trigger Window" means the 24-month period
commencing on the date immediately following the Effective Time and means the
24-month period commencing on the date that a Subsequent Change of Control
occurs."

          2.  Paragraph 1 is amended to delete the last sentence of subparagraph
(o).

          3.  Paragraph 1 is amended to renumber subparagraph (g) as (h), to
renumber subparagraph (h) as (j), to renumber subparagraphs (i) through (l) as
(l) through (o), and to renumber subparagraphs (m) through (o) as (q) through
(s), and to add new subparagraphs (g), (i), (k) and (p) to read as follows:

              "(g) "Effective Time" shall be ascribed the meaning set forth for
such term on Annex A attached hereto.

              (i)  "Merger" shall be ascribed the meaning set forth for such
term on Annex A attached hereto.

              (k)  "Parent Company" shall be ascribed the meaning set forth for
such term on Annex A attached hereto.

              (p)  "Subsequent Change of Control" shall be ascribed the meaning
set forth for such term on Annex A attached hereto."

          When amending sections or subsections currently set forth in the Plan,
the section or subsection references below refer to the sections and subsections
as they existed prior to the renumbering as provided for above.  Moreover, all
current references in the Plan to other sections and subsections of the Plan
shall be deemed to refer to the applicable section or subsection as renumbered
herein.

          4.  Paragraph 2 is amended to add "; or" at the end of subparagraph
(b)(i)(B)(II) and to add a new subparagraph (b)(i)(B)(III) which shall read as
follows:

              "(III)  with respect to Tier 1 and 2 Employees of the Company, a
demotion to a lesser job."


          5.  Paragraph 2 is amended to eliminate subparagraph (b)(ii).

          6.  Paragraph 2 is amended to eliminate the parenthetical "(excluding
an ARCO Acquisition)" from subparagraph (d).

          7.  The reference in Paragraph 3(b) to "Section 1(o)" is deleted and
replaced with "Paragraph 1(s)."

          8.  Paragraph 3 is amended to delete the second sentence of
subparagraph (c).

          9.  Paragraph 3 is amended to eliminate subparagraph (d).

          10. Paragraph 4 is amended in its entirety and shall now read as
follows:

              "4. AUTHORIZED OFFICER

                  Both the Chief Financial Officer and the Vice President, Human
Resources of the Company whose duties with respect to this Plan are provided
herein, will be those persons who have been employed in such positions by the
Company since April 1, 1999.  Any action required to be taken under this Plan by
the Chief Financial Officer and the Vice President, Human Resources of the
Company may be taken by either individual if the other position is vacant.  If
the positions of Chief Financial Officer and Vice President, Human Resources of
the Company are both vacant, actions required to be taken under the Plan by such
officers may only be taken by one of the officers of the Company set forth below
(in the order stated) who was in such position on April 1, 1999:

                            (a)  Controller
                            (b)  General Counsel
                            (c)  Senior Vice President of Production
                            (d)  Senior Vice President of Exploration
                            (e)  Vice President of Business Development.

If none of such persons is in such position, the Special Plan Administrator
shall take actions required to be taken under the Plan."

          11. Annex A is amended, with respect to the definition of "Change of
Control" and the definition of "Outside Director", to replace the terms
"Atlantic Richfield Company" and "ARCO" with the term "Parent Company".  This
amendment shall be construed consistently with the intent that the Merger, if it
occurs, is a Change of Control.

          12. Annex A, with respect to the definition of "Change of Control," is
amended, effective as of the Effective Time, to add "; or" at the end of
Paragraph (6) and to add new Paragraph (7) which shall read as follows:

              "(7) A Subsequent Change of Control."


     13.  Annex A is amended to add the following definitions:

          ""Effective Time" means, with respect to the Merger, such time as the
Certificate of Merger is duly filed with the Secretary of State of the State of
Delaware or at such later time as is specified in the Certificate of Merger in
accordance with the Delaware General Corporation Law.

          "Merger" shall mean the merger between Atlantic Richfield Company
Prairie Holdings, Inc., a subsidiary of BP Amoco p.l.c.

          "Parent Company" shall mean, until the Effective Time of the Merger,
Atlantic Richfield Company and, after the Effective Time of the Merger, BP Amoco
p.l.c. or any of its subsidiaries or affiliates and any of their respective
successors.

          "Subsequent Change of Control" shall mean each Change of Control which
occurs after the consummation of a transaction constituting a Change of
Control."

     14.  The Plan is amended to replace (except in Paragraph 4(b) of the Plan
as amended above) the term "General Counsel" with the term "Vice President,
Human Resources."

     15.  The Plan is amended to add Paragraph 5 which shall read as follows:

     "5.  Retiree Medical Coverage.

     A Retiree (as hereafter defined) may continue participation in this Plan
provided the Retiree waives the continuation coverage to which he or she may be
entitled to under COBRA.  The term "Retiree" shall mean any Participant who was
participating in the Plan and who while a participating in the Plan:

     (a)  Terminated employment and, at such time, was entitled to an immediate
     retirement allowance from a qualified retirement plan of the Company
     (excluding any enhanced retirement allowance payable immediately upon
     termination of employment in connection with a Change of Control of the
     Company, such as is contained in Section 30 of the Vastar Resources, Inc.
     Retirement Plan); or

     (b)  Terminated employment after attainment of age 53 years plus six months
     and ten or more years of Membership Service, as defined in the Vastar
     Resources, Inc. Retirement Plan and is eligible for an allowance under the
     Vastar Resources, Inc.  Special Termination Allowance Plan.  A Participant
     described in this subparagraph (b) shall become a Retiree upon the date the
     Participant becomes


     eligible to receive an immediate monthly retirement allowance from a
     qualified retirement plan of the Company (excluding any enhanced retirement
     allowance payable immediately upon termination of employment in connection
     with a Change of Control of the Company, such as is contained in Section 30
     of the Vastar Resources, Inc. Retirement Plan); or

     (c) Terminated employment in accordance with the terms and conditions of a
     written agreement between the Company and the Employee, which grants the
     Employee Retiree status.  The applicable provisions of such written
     agreement are incorporated by reference as a provision of this Plan;

     (d)  (i) Has a termination of employment as described in Paragraph 2(b)(i)
     of the Plan during a Benefit Trigger Window and the Participant (A) does
     not elect immediate receipt of the enhanced component under Paragraph 30.5
     of the Vastar Resources, Inc. Retirement Plan, (B) as of the date of the
     Participant's termination of employment, has attained the age of 48 years
     plus six months or more and (C) as of the date of the Participant's
     termination of employment, has five or more years of Membership Service, as
     defined under the Vastar Resources, Inc. Retirement Plan and is eligible
     for an allowance under the Vastar Resources, Inc. Special Termination
     Allowance Plan. A Participant described in this subparagraph (d)(i) shall
     become a Retiree on the earliest of the 19th month after such Participant's
     termination of employment or the date the Participant becomes eligible to
     receive an immediate monthly retirement allowance under the Vastar
     Resources, Inc. Retirement Plan (other than the allowance described in
     Paragraph 30.5 of such plan).

     (ii) Has a termination of employment as described in Paragraph 2(b)(i) of
     the Plan during a Benefit Trigger Window and the Participant, at such time,
     (A) has attained the age of 48 years plus six months or more and (B) has
     ten or more years of Membership Service, as defined in the Vastar
     Resources, Inc. Retirement Plan; provided that the Participant is eligible
     for an allowance under the Vastar Resources, Inc. Special Termination
     Allowance Plan. A Participant described in this subparagraph (d)(ii) shall
     become a Retiree on the earliest of the 19th month after such Participant's
     termination of employment or the date the Participant becomes eligible to
     receive an immediate monthly retirement allowance under the Vastar
     Resources, Inc. Retirement Plan (other than the allowance described in
     Paragraph 30.5 of such plan)."

     16.  All amendments shall be effective as of July 21, 1999 unless otherwise
noted.


     Executed as of the 12th day of August, 1999.


     ATTEST:                                 VASTAR RESOURCES, INC.


     By: /s/ Jonathan D. Edelfelt            By: /s/ Jeffrey M. Bender
        -------------------------------         --------------------------------
          Jonathan D. Edelfelt                   Jeffrey M. Bender
          Associate Secretary                    Vice President, Human Resources



APPROVED as to form this 12th day
of August, 1999

WACHOVIA BANK, N.A.,


By: /s/ Peter D. Quinn
   --------------------------
Name: Peter D. Quinn
     ------------------------
Title: Senior Vice President
      -----------------------