Du Pont Co., whose proposed combination with Conoco Inc. will result in the seventh-biggest industrial company in the United States, may have been prompted in part by a relaxed attitude on the part of antitrusters in the Reagan administration.

But things were not so relaxed more than 20 years ago during the Truman administration, which took Du Pont all the way to the Supreme Court to force the Company to give up its holdings in General Motors Corp.

Under its founder, William C. Durant, GM and its predecessor companies several times fell on hard times. After the banks bailed it out in 1910, they forced Durant aside and ran the company themselves until 1915. Pierre S. du Pont, then president of the giant chemical concern (which in later years capitalized the "D" in its name), had bought 2,000 shares of GM the year before on the advice of John J. Raskob, his personal financial adviser and company treasurer.

In November 1915, du Pont was elected chairman of GM. Durant regained control as president the following year. The chemical company invested heavily in GM, as did J. P. Morgan & Co., the big bank. When GM got into trouble again in 1920 and Durant got himself into financial difficulties trading GM stock, du Pont again came to the rescue. Du Pont holdings in GM climbed as high as 36 percent of the stock.

In 1949, the Truman administration filed suit against du Pont and General Motors, charging that the du Pont investment in GM violated antitrust laws. A lower court disagreed with the administration, but the Supreme Court reversed the ruling in 1957.

Du Pont interests, the company and the family members, were forced to divest themselves of GM stock, but not until Congress approved a massive Tax break, saving du Pont from much of the capital gains tax liability stemming from the stock sale.