SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549
                                   ----------
                                   SCHEDULE TO
            Tender Offer Statement Under Section 14(d)(1) or 13(e)(1)
                     of the Securities Exchange Act of 1934

                                (Amendment No. 3)

                                   ----------
                               PRIMARK CORPORATION
                       (Name of Subject Company (Issuer))
                                   ----------
                         MARQUEE ACQUISITION CORPORATION
                             THE THOMSON CORPORATION
                      (Names of Filing Persons (Offerors))
                                   ----------
                      Common Stock, No Par Value Per Share
                         (Title of Class of Securities)
                                   ----------
                                    741903108
                      (CUSIP Number of Class of Securities)

                             Michael S. Harris, Esq.
                             The Thomson Corporation
                        Metro Center at One Station Place
                           Stamford Connecticut 06902
                            Telephone (203) 969-8700

  (Name, Address and Telephone Number of Persons Authorized to Receive Notices
                 and Communications on Behalf of filing persons)

                                   ----------
                                    Copy to:
                             David W. Heleniak, Esq.
                               Shearman & Sterling
                              599 Lexington Avenue
                            New York, New York 10022
                                 (212) 848-4000

                            CALCULATION OF FILING FEE
================================================================================
        Transaction Valuation*                   Amount of Filing Fee**
- --------------------------------------------------------------------------------
             $913,518,708                              $182,703.74
================================================================================

*    Estimated for purposes of calculating the amount of the filing fee only.
     Calculated by multiplying $38.00, the per share tender offer price, by
     24,039,966, the sum of the 20,308,103 currently outstanding shares of
     Common Stock sought in the Offer and the 3,731,863 shares of Common Stock
     subject to options that vested as of June 12, 2000.
**   Calculated as 1/50 of 1% of the transaction value.
|x|  Check the box if any part of the fee is offset as provided by Rule
     0-11(a)(2) and identify the filing with which the offsetting fee was
     previously paid. Identify the previous filing by registration statement
     number, or the Form or Schedule and the date of its filing.

Amount Previously Paid:   $182,703.74     Filing Party: The Thomson Corporation,
                                                        Marquee Acquisition
                                                        Corporation
Form or Registration No.: Schedule TO     Date Filed:   June 14, 2000
|_|  Check the box if the filing relates solely to preliminary communications
     made before the commencement of a tender offer.
Check the appropriate boxes to designate any transactions to which the statement
relates:
|X|  third-party tender offer subject to Rule 14d-1.
|_|  issuer tender offer subject to Rule 13e-4.
|_|  going-private transaction subject to Rule 13e-3.
|X|  amendment to Schedule 13D under Rule 13d-2.
Check the following box if the filing is a final amendment reporting the results
of the tender offer: |_|

                                       1



         This Amendment No. 3 (the "Amendment") amends and supplements the
Tender Offer Statement on Schedule TO filed with the Securities and Exchange
Commission on June 14, 2000 (the "Schedule TO"), by Marquee Acquisition
Corporation, a Michigan corporation ("Purchaser") and an indirect wholly owned
subsidiary of The Thomson Corporation, a corporation organized under the laws of
Ontario, Canada ("Thomson"). The Schedule TO relates to the offer by Purchaser
to purchase all outstanding shares of Common Stock, no par value, including
associated common stock purchase rights (together, the "Shares"), of Primark
Corporation, a Michigan corporation (the "Company"), at a purchase price of
$38.00 per Share, net to each seller in cash, upon the terms and subject to the
conditions set forth in the Offer to Purchase dated June 14, 2000 (the "Offer to
Purchase") and in the related Letter of Transmittal, copies of which are filed
as Exhibits (a)(1) and (a)(2) to the Schedule TO (which, together with any
amendments or supplements thereto, collectively constitute the "Offer").
Capitalized terms used and not defined herein shall have the meanings ascribed
to such terms in the Offer to Purchase or in the Schedule TO.

Item 4.  Terms of the Transaction.

         Item 4 of the Schedule TO is hereby amended and supplemented by the
following:

         On July 12, 2000, Thomson issued a press release announcing the
extension of the Offer until 12:00 midnight, Eastern Daylight Time, on Friday,
August 4, 2000. Thomson also announced that it had been advised by Morgan
Stanley Dean Witter, the dealer manager for the Offer, that as of 5:00 p.m.,
EDT, on July 11, 2000, approximately 8,445,103 Common Shares had been tendered.
The foregoing description of the press release which is attached hereto as
Exhibit (a)(11) is incorporated by reference herein.


Item 12. Materials to Be Filed as Exhibits

         Item 12 of the Schedule TO is hereby amended and supplemented to
include the following information:

(a)(11)  Press Release issued by Thomson on July 12, 2000


                                       2



         After due inquiry and to the best of my knowledge and belief, I certify
that the information set forth in this statement is true, complete and correct.

Dated:  July 13, 2000

                                       MARQUEE ACQUISITION CORPORATION


                                       By:   /s/ Michael S. Harris
                                            --------------------------------
                                            Name:  Michael S. Harris
                                            Title: Vice President



                                       THE THOMSON CORPORATION


                                       By:   /s/ Michael S. Harris
                                            --------------------------------
                                            Name:  Michael S. Harris
                                            Title: Senior Vice President and
                                                   General Counsel






                                       3



                                  EXHIBIT INDEX






Exhibit No.
(a)(1)     Offer to Purchase, dated June 14, 2000.*
(a)(2)     Form of Letter of Transmittal.*
(a)(3)     Form of Notice of Guaranteed Delivery.*
(a)(4)     Form of Letter to Brokers, Dealers, Commercial Banks, Trust Companies
           and Other Nominees.*
(a)(5)     Form of Letter from Brokers, Dealers, Commercial Banks, Trust
           Companies and Nominees to Clients.*
(a)(6)     Form of Guidelines for Certification of Taxpayer Identification
           Number on Substitute Form W-9.*
(a)(7)     Summary Advertisement as published in The Wall Street Journal on June
           14, 2000.*
(a)(8)     Joint Press Release issued by Thomson and the Company on June 5,
           2000.*
(a)(9)     Press Release issued by Thomson on June 22, 2000.**
(a)(10)    Joint Press Release issued by Thomson and the Company on June 28,
           2000.***
(a)(11)    Press Release issued by Thomson on July 12, 2000.
(b)        None.
(c)        None.
(d)(1)     Agreement and Plan of Merger, dated as of June 5, 2000, among
           Thomson, Purchaser and the Company.*
(d)(2)     Confidentiality Agreement dated April 4, 2000, between Thomson and
           the Company.*
(d)(3)     Shareholders Agreement, dated June 5, 2000, among Thomson, Purchaser,
           Joseph E. Kasputys, Stephen H. Curran and Michael R. Kargula.*
(d)(4)     Guarantee, dated June 5, 2000, of Thomson in favor of Joseph E.
           Kasputys.*
(d)(5)     Letter Agreement, dated June 5, 2000, between Primark Corporation and
           Stephen H. Curran.*
(d)(6)     Letter Agreement, dated June 5, 2000, between Primark Corporation and
           Michael R. Kargula.*
(d)(7)     Letter Agreement, dated June 5, 2000, between Primark Corporation and
           Joseph E. Kasputys.*
(g)        None.
(h)        None.

- -----------

*   Incorporated by reference to Thomson's Schedule TO, filed June 14, 2000.

**  Incorporated by reference to Thomson's Schedule TO/A, filed June 23, 2000

*** Incorporated by reference to Thomson's Schedule TO/A, filed June 28, 2000.


                                       4

Exhibit (a)(11) - Press Release issued by Thomson on July 12, 2000.


The Thomson Corporation                                     [LOGO]
Toronto Dominion Bank Tower, Suite 2706
PO Box 24, Toronto-Dominion Centre
Toronto, Ontario M5K 1A1
Tel (416) 360-8700   Fax (416) 360-8812
www.thomson.com


News Release


Investor Contact:                       Media Contact:
- ----------------                        -------------
John Kechejian                          Janey Loyd
Vice President, Investor Relations      Vice President, Corporate Communications
(203) 969-8700                          (203) 969-8700
john.kechejian@thomson.com              janey.loyd@thomson.com



For Immediate Release
- --------------------------------------------------------------------------------

                         THE THOMSON CORPORATION EXTENDS
                  ITS EQUITY AND DEBT TENDER OFFERS FOR PRIMARK

- --------------------------------------------------------------------------------

         TORONTO, ONTARIO, July 12, 2000 - The Thomson Corporation (TSE: TOC)
announced today that Marquee Acquisition Corporation ("Marquee"), a wholly owned
subsidiary of Thomson, is extending its offer to acquire all outstanding shares
of common stock of Primark Corporation until Friday, August 4, 2000 at 12:00
midnight Eastern Daylight Time (EDT). The offer to acquire the common stock of
Primark was previously scheduled to expire at 12:00 midnight EDT on Wednesday,
July 12, 2000.

         In addition, Thomson announced that Marquee is extending its offer to
acquire all of the outstanding Primark 9 1/4% Senior Subordinated Notes due 2008
(the "Notes") until Monday, August 7, 2000 at 12:00 midnight EDT. The offer to
acquire the Notes was previously scheduled to expire at 5:00 p.m. EDT on
Thursday, July 20, 2000.

         Thomson and Primark previously announced that they had each received
requests for additional information and documentary material from the Antitrust
Division of the Department of Justice (DOJ) following the DOJ's routine review
of the proposed acquisition of Primark by Thomson. The waiting period under the
Hart-Scott-Rodino Act will expire ten days after Thomson substantially complies
with the DOJ's request.

         Based on the latest count of tendered shares and Notes, as of 5:00 p.m.
EDT on July 11, 2000, 8,445,103 shares of Primark common stock (constituting
41.6% of the outstanding common stock) had been tendered and not withdrawn, and
US$149.9 million in the aggregate principal amount of the Notes (constituting
99.9% of the outstanding Notes) had been tendered and not withdrawn.

         After the receipt by Marquee of the requisite consents pursuant to its
Offer to Purchase the Notes and related Consent Solicitation, Primark and State
Street Bank and Trust Company, as trustee, have executed a supplemental
indenture today amending the indenture dated December 21, 1998 governing the
Notes. The provisions of the supplemental indenture will not become operative
unless and until Marquee accepts for payment the Notes that have been validly
tendered and not withdrawn prior to August 7, 2000.



         The tender offer for the Primark common stock is being made through,
and the foregoing announcement is qualified in its entirety by reference to,
Marquee's Offer to Purchase dated June 14, 2000, and the related letter of
transmittal. The tender offer for the Notes is being made through, and the
foregoing announcement is qualified in its entirety by reference to, Marquee's
Offer to Purchase and Consent Solicitation Statement dated June 22, 2000, and
the related letter of transmittal. Primark stockholders and noteholders should
read the applicable Offer to Purchase and related letter of transmittal in their
entirety prior to making any decision as to the respective tender offers.

         The Thomson Corporation (www.thomson.com), with 1999 revenues of US$5.8
billion, is a leading, global e-information and solutions company in the
business and professional marketplace. Thomson's common shares are listed on the
Toronto and London Stock Exchanges.

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