EXHIBIT 4


                    AMENDMENT dated as of March 31, 1999, to
               the Rights Agreement dated as of July 24, 1995
               (the "Rights Agreement"), between ATLANTIC
               RICHFIELD COMPANY (the "Company") and FIRST
               CHICAGO TRUST COMPANY OF NEW YORK, as Rights
               Agent (the "Rights Agent").


          Pursuant to the terms of the Rights Agreement and
in accordance with Section 27 thereof, the following actions
are hereby taken prior to executing the Merger Agreement and
Stock Option Agreement referred to below:

          Section 1. Amendments to Rights Agreement. The
Rights Agreement is hereby amended as follows:

          (a) The definition of "Acquiring Person" in Section
1 of the Rights Agreement is amended to add the following
sentence at the end thereof:

          "Notwithstanding anything in this Rights Agreement
     to the contrary, none of BP Amoco p.l.c., Prairie
     Holdings, Inc., or any Affiliate or Associate of either
     shall be deemed to be an Acquiring Person, either
     individually or collectively, solely by virtue of (i)
     the announcement of the Merger (as such term is defined
     in the Merger Agreement), (ii) the acquisition of Common
     Shares of the Company pursuant to the Merger or the
     Stock Option Agreement, (iii) the execution of the
     Merger Agreement or the Stock Option Agreement or (iv)
     the consummation of the Merger or of the other
     transactions contemplated in the Merger Agreement or the
     Stock Option Agreement."

          (b) The following definitions shall be added to
Section 1 of the Rights Agreement:

          "'Merger Agreement' shall mean the Agreement and
     Plan of Merger dated as of March 31, 1999 among BP Amoco
     p.l.c., Prairie Holdings, Inc., and the Company."

          "'Stock Option Agreement' shall mean the Stock
     Option Agreement dated as of March 31, 1999 between the
     Company and BP Amoco p.l.c."

          (c) Section 3(a) of the Rights Agreement is amended
to add the following sentence at the end thereof:

          "Notwithstanding anything in this Rights Agreement
     to the contrary, a Distribution Date shall not be deemed
     to have occurred solely as the result of (i) the





         announcement of the Merger, (ii) the acquisition of
         Common Shares of the Company pursuant to the Merger or
         the Stock Option Agreement, (iii) the execution of the
         Merger Agreement or the Stock Option Agreement or
         (iv) the consummation of the Merger or of the other
         transactions contemplated in the Merger Agreement or
         the Stock Option Agreement.";

          (d) Section 7(a) of the Rights Agreement is amended
to replace the words "at or prior to the earliest of (i) the
Close of Business on the Final Expiration Date, (ii) the
Redemption Date, or (iii) the time at which such Rights are
exchanged as provided in Section 24 hereof" in Section 7(a)
with the following: "at or prior to the earlier of (i) the
Effective Time of the Merger (as defined in the Merger
Agreement) or (ii) the Close of Business on the Final
Expiration Date".

          Section 2. Full Force and Effect. Except as
expressly amended hereby, the Rights Agreement shall continue
in full force and effect in accordance with the provisions
thereof on the date hereof.

          Section 3. Governing Law. This Amendment shall be
governed by and construed in accordance with the law of the
State of Delaware applicable to contracts to be made and
performed entirely within such State.


          IN WITNESS WHEREOF, the Company and the Rights
Agent have caused this Amendment to be duly executed as of
the day and year first above written.


                              ATLANTIC RICHFIELD COMPANY,

                                by:
                                   -------------------------
                                   Name:
                                   Title:


                              FIRST CHICAGO TRUST COMPANY
                              OF NEW YORK, as Rights Agent

                                by:
                                   -------------------------
                                   Name:
                                   Title: