Du Pont Co., the nation's largest chemical company, announced today that it had filed a formal registration statement with the Securities and Exchange Commission to set the motion its offer to buy Conoco Inc. for $7.3 billion.

It will be the largest corporate merger in history if it goes through.

On Monday, Du Pont and Conoco startled the corporate world by announcing that both boards of directors had approved a friendly takeover of Conoco by Du Pont, a merger that would result in the seventh-largest industrial corporation in the United States, with combined 1980 sales of about $32 billion.

The merger also appeared to preempt a bid by Seagram Co. Ltd., the giant Montreal distiller, to buy 41 percent, and effective control, of Conoco, the nation's ninth-biggest oil company and 14th-biggest industrial company.

Seagram, which earlier this year lost a bid to buy St. Joe Minerals Corp., has had no comment on the proposed Du Pont-Conoco marriage.

Du Pont said that First Boston Corp., a New York investment banking firm, will act as dealer and manager of the offer. Du Pont has said it will pay $3 billion to buy up to 34.4 million, or about 40 percent, of Conoco's outstanding shares and will exchange 1.6 shares of Du Pont stock for each of the remaining 52 million shares.

The $7.3 billion price tag was based on Du Pont's closing last Thursday of $51.25 a share. Today Du Pont closed off 12 1/2 cents to $46.25. At that price the deal is worth about $6.85 billion. Conoco, which gained more than $7 a share Monday (Du Pont lost nearly $5), closed down $1 to $75.50 on the New York Stock Exchange today.

Du Pont said its offer is dependent on it receiving more than half the outstanding Conoco shares and on approval by Du Pont shareholders at a special meeting scheduled for mid-August.

Seagram's $73-a-share tender offer remains outstanding. The company has until July 17 to withdraw its offer to Conoco shareholders. The offer itself is scheduled to expire July 24.