Filed by Sanofi-Synthélabo
Pursuant to Rule 165 and Rule 425(a) under the United States Securities Act of 1933,
as amended, and deemed filed pursuant to Rule 14d-2(b)(2) of the
United States Securities Exchange Act of 1934, as amended

Subject Company: Aventis
Commission File No. 001-10378
Date: April 8, 2004

     The following is an English translation of materials presented on April 8, 2004 by Mr. Jean François-Dehecq, Chairman and Chief Executive Officer of Sanofi-Synthelabo, to the French Group Council of Aventis (le comité de groupe d’Aventis), the representative body of Aventis’s French employees constituted pursuant to Article 439-1 of the French Labor Code.

     In connection with the proposed acquisition of Aventis, Sanofi-Synthélabo has filed with the United States Securities and Exchange Commission (SEC), a registration statement on Form F-4 (File no: 333-112314), which includes a preliminary prospectus and related exchange offer materials, to register the Sanofi-Synthélabo ordinary shares (including Sanofi-Synthélabo ordinary shares represented by Sanofi-Synthélabo ADSs) to be issued in exchange for Aventis ordinary shares held by holders located in the United States and for Aventis ADSs held by holders wherever located. At the appropriate time, Sanofi-Synthélabo will file a Statement on Schedule TO with the SEC. Investors and holders of Aventis securities are strongly advised to read the registration statement and the preliminary prospectus, the related exchange offer materials and the final prospectus and the Statement on Schedule TO (when available), and any other relevant documents filed with the SEC, as well as any amendments and supplements to those documents, because they will contain important information. Investors and holders of Aventis securities may obtain free copies of the registration statement, the preliminary prospectus and related exchange offer materials, and the final prospectus and Statement on Schedule TO (when available), as well as other relevant documents filed with the SEC, at the SEC’s web site at www.sec.gov and will receive information at the appropriate time on how to obtain transaction-related documents for free from Sanofi-Synthélabo or its duly designated agent.

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Important Information

In connection with the proposed acquisition of Aventis, Sanofi-Synthélabo has filed with the United States Securities and Exchange Commission (SEC), a registration statement on Form F-4 (File no: 333-112314), which includes a preliminary prospectus and related exchange offer materials, to register the Sanofi-Synthélabo ordinary shares (including Sanofi-Synthélabo ordinary shares represented by Sanofi-Synthélabo ADSs) to be issued in exchange for Aventis ordinary shares held by holders located in the United States and for Aventis ADSs held by holders wherever located. At the appropriate time, Sanofi-Synthélabo will file a Statement on Schedule TO with the SEC. Investors and holders of Aventis securities are strongly advised to read the registration statement and the preliminary prospectus, the related exchange offer materials and the final prospectus and the Statement on Schedule TO (when available), and any other relevant documents filed with the SEC, as well as any amendments and supplements to those documents, because they will contain important information. Investors and holders of Aventis securities may obtain free copies of the registration statement, the preliminary prospectus and related exchange offer materials, and the final prospectus and Statement on Schedule TO (when available), as well as other relevant documents filed with the SEC, at the SEC’s web site at www.sec.gov and will receive information at the appropriate time on how to obtain transaction-related documents for free from Sanofi-Synthélabo or its duly designated agent.

The public offer to holders of Aventis ordinary shares located in Germany (the “German Offer”) is being made in accordance with applicable German law and pursuant to an offer document/sales prospectus, which is available free of charge at BNP Paribas Securities Services, Grüneburgweg 14, D-60322 Frankfurt am Main (Fax: 069 — 152 05 277) and on the website of the Company (www.sanofi-synthelabo.com). Any decision to tender Aventis ordinary shares in exchange for Sanofi-Synthélabo ordinary shares under the German Offer must be taken exclusively with regard to the terms and conditions of the German Offer, as well as with regard to the information included in the offer document/sales prospectus, including any amendments thereto, issued in Germany.

This document does not constitute an offer to purchase or exchange or the solicitation of an offer to sell or exchange any securities of Aventis or an offer to sell or exchange or the solicitation of an offer to buy or exchange any securities of Sanofi-Synthélabo, nor shall there be any sale or exchange of securities in any jurisdiction (including the United States, Germany, Italy and Japan) in which such offer, solicitation or sale or exchange would be unlawful prior to the registration or qualification under the laws of such jurisdiction. The distribution of this communication may, in some countries, be restricted by law or regulation. Accordingly, persons who come into possession of this document should inform themselves of and observe these restrictions. The solicitation of offers to buy Sanofi-Synthélabo ordinary shares (including Sanofi-Synthélabo ordinary shares represented by Sanofi-Synthélabo ADSs) in the United States will only be made pursuant to a prospectus and related offer materials that Sanofi-Synthélabo expects to send to holders of Aventis securities. The Sanofi-Synthélabo ordinary shares (including Sanofi-Synthélabo ordinary shares represented by Sanofi-Synthélabo ADSs) may not be sold, nor may offers to buy be accepted, in the United States prior to the time the registration statement becomes effective. No offering of securities shall be made in the United States except by means of a prospectus meeting the requirements of Section 10 of the United States Securities Act of 1933, as amended. In France, holders of Aventis securities are requested, with respect to the offer, to refer to the prospectus (note d’information), which has been granted visa number 04-0090 by the Autorité des marchés financiers (“AMF”) and which is available on the website of the AMF (www.amf-france.org) and without cost from: BNP Paribas Securities Services, GIS-Emetteurs, Service Logistique, Les Collines de l’Arche, 75450 Paris Cedex 9.

Forward-Looking Statements

This press release contains statements that constitute forward-looking statements within the meaning of the U.S. Private Securities Litigation Reform Act of 1995. Forward-Looking statements are statements that are not historical facts. These statements include financial projections and estimates and their underlying assumptions, statements regarding plans, objectives and expectations with respect to future operations, products and services, and statements regarding future performance. Forward-looking statements are generally identified by the words “expect,” “anticipates,” “believes,” “intends,” “estimates” and similar expressions. Although Sanofi-Synthélabo’s management believes that the expectations reflected in such forward-looking statements are reasonable, investors are cautioned that forward-looking information and statements are subject to various risks and uncertainties, many of which are difficult to predict and generally beyond the control of Sanofi-Synthélabo, that could cause actual results and developments to differ materially from those expressed in, or implied or projected by, the forward-looking information and statements. The following factors, among other risks and uncertainties that are described in our Form 20-F as filed with the SEC on April 2, 2004 and in the Reference Document filed with the French Commission des Opérations de Bourse (now the Autorité des Marchés Financiers) on April 2, 2004, could cause actual results to differ materially from those described in the forward-looking statements: the ability of Sanofi-Synthélabo to expand its presence profitably in the United States; the success of Sanofi-Synthélabo’s research and development programs; the ability of Sanofi-Synthélabo to protect its intellectual property rights; and the risks associated with reimbursement of health care costs and pricing reforms, particularly in the United States and Europe. Sanofi-Synthélabo does not undertake any obligation to provide updates or to revise any forward-looking statements.

Investors and security holders may obtain a free copy of the Form 20-F and any other documents filed by Sanofi-Synthélabo with the SEC at www.sec.gov as well as of the Reference Document filed with the French Autorité des Marchés Financiers at www.amf-france.org or directly from Sanofi-Synthélabo on our web site at: www.sanofi-synthelabo.com.