UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

(Amendment No. 1)

Tender Offer Statement under Section 14(d)(1) or 13(e)(1)
of the Securities Exchange Act of 1934

Aventis

Sanofi-Synthelabo

ISIN No. FR0000130460

053561106

Copies to:

Table of Contents

     This Amendment No. 1 (the “Amendment”) amends and supplements the Tender Offer Statement on Schedule TO (the “Schedule TO”) filed initially with the United States Securities and Exchange Commission on April 12, 2004 by Sanofi-Synthelabo, a French société anonyme, in connection with Sanofi-Synthelabo’s offer to acquire (1) all ordinary shares, nominal value €3.82 per share, of Aventis, a French société anonyme, that are held by holders who are located in the United States and (2) all American Depositary Shares of Aventis (each an “Aventis ADS”, and each Aventis ADS representing one Aventis ordinary share), held by holders wherever located, pursuant to the terms and subject to the conditions set forth in the Prospectus, dated April 9, 2004 (the “Prospectus”), and the related ADS Letter of Transmittal and Form of Acceptance (collectively, and as hereafter amended or supplemented, the “U.S. Offer”). Capitalized terms not otherwise defined herein shall have the meanings ascribed to them in the Schedule TO.

Item 8. Interests in Securities of the Subject Company

     Item 8 is amended and supplemented to add the following information:

     Jean-Pierre Meyers, Vice Chairman of the Board of Directors of L’Oréal, is also Vice Chairman (“Vice-Président du Conseil d’Administration”) of Fondation Bettencourt Schueller. Liliane Bettencourt, a member of the board of directors of L’Oréal, is also Chairman (“Président du Conseil d’Administration”) of Fondation Bettencourt Schueller, Gespral S.A. and Clymène S.A.. As of the date hereof, Fondation Bettencourt Schueller owns 3,745 Aventis ordinary shares; Gespral S.A. owns 4,400 Aventis ordinary shares; and Clymène S.A. owns 2,000 Aventis ordinary shares. On January 28, 2004, Clymène S.A. purchased 500 Aventis ordinary shares at an average purchase price of €58.30. The Aventis ordinary shares were purchased by an agent through a discretionary mandate.

     On February 13, 2004, François Vachey, Executive Vice President of Human Resources of L’Oréal, sold 705 Aventis ordinary shares for an average price of €62 per share in an open market transaction effected through his account with BNP Paribas and sold an additional 140 Aventis ordinary shares for an average price of €62 per share in an open market transaction effected through his plan d’épargne en actions account.

Item 11. Additional Information

     Item 11 is amended and supplemented to include the following information:

     On April 19, 2004, Aventis filed a complaint in the United States District Court for the District of New Jersey alleging that the public filings and public statements of Sanofi-Synthelabo in connection with the U.S. Offer violate Section 14(d) and Section 14(e) of the Williams Act, 15 U.S.C. §§ 78n(d)&(e). In the complaint, Aventis seeks (1) a declaration that Sanofi-Synthelabo has violated Section 14(d) and Section 14(e) of the Williams Act and a judgment requiring Sanofi-Synthelabo to make prompt corrective disclosures, (2) a preliminarily and permanent injunction prohibiting Sanofi-Synthelabo, its agents and employees from further violating Section 14(d) and Section 14(e) of the Williams Act, (3) a preliminary and permanent injunction prohibiting Sanofi-Synthelabo, its agents and employees from soliciting the tender of any Aventis shares unless and until Sanofi-Synthelabo fully complies with all applicable provisions of the federal securities laws and unless and until the court determines that the effects of Sanofi-Synthelabo’s alleged unlawful conduct have dissipated, (4) an award to Aventis of the costs and disbursements of the litigation and (5) an award to Aventis of any other relief that the court may deem just and proper. Sanofi-Synthelabo believes that the allegations in the complaint are without merit and plans to vigorously contest and defend against the claims made by Aventis. A copy of Aventis’s complaint is filed as Exhibit (a)(5)(vi) hereto.

     In connection with this complaint, Aventis also filed on April 19, 2004 a motion for an order (1) granting Aventis leave to conduct certain expedited discovery in support of its request for declaratory and injunctive relief and (2) setting aside the automatic stay of discovery provided by the Private Securities Litigation Reform Act insofar as that stay may apply to the litigation.

Item 12. Exhibits

     Item 12 is amended and supplemented to include the following exhibits:

SIGNATURE

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

Exhibit Index