The Reagan administration plans to announce major restrictions next week on toxaphene, once the most heavily used pesticide in the country.

The chemical, considered a major cancer threat, has been used mainly on cotton fields in the South, but it has recently been discovered in fish in the Great Lakes, prompting calls for its cancellation. Scientists have said that winds apparently carried the chemical across the country and that it mixed with rainfall in the Midwest.

The Environmental Protection Agency is to announce the restrictions at a news conference Monday. The agency will ban some major uses of the pesticide, but will allow some "emergency" and minor uses, according to Maureen Hinkle of the National Audubon Society, who obtained details of the decision from agency officials.

Hinkle said the spraying of cotton crops will be banned, apparently out of concern for the movement of the chemical through the atmosphere. Minor uses that will still be allowed include the dipping of cows and sheep to kill parasites and applications to certain tropical fruits -- for example, to bananas in the Virgin Islands and Puerto Rico, where weevils are a common pest.

EPA officials declined to comment on the planned restrictions, saying they will be revealed on Monday by John Todhunter, assistant EPA administrator for pesticides and toxic substances. EPA officials told some groups of the decision yesterday; among them was the Audubon Society.

EPA has been considering a toxaphene ban for more than five years, based on studies that call the chemical a threat to health and the environment. At the time the EPA review began, more than 100 million pounds of the pesticide were used each year, accounting for about one-fifth of all pesticide uses in the country.

Toxaphene use increased sharply after the banning of DDT in 1972, but has recently fallen to about 16 million pounds a year as insects have apparently developed an immunity to it. Its major use has been as a spray for pests that attack cotton and soybean crops in the South, but it also has hundreds of minor uses.

The EPA review was loudly criticized at first by farmers and by chemical manufacturers, who questioned the validity of studies linking toxaphene to cancer in laboratory mice and rats. But subsequent studies, including tests by the National Cancer Institute, produced the same results.

EPA officials reportedly began leaning more strongly toward restricting the use of toxaphene after it was found in fish in Lake Michigan--evidence that it could travel long distances through the atmosphere without decomposing, and that it could enter the food chain. Like DDT, toxaphene does not easily decompose or lose its harmful effects.

Pressure also came from Congress, which voted last month to provide funds for EPA to start procedures to ban toxaphene unless the agency reached a decision on the chemical in 60 days.

In an emotional speech on the House floor, Rep. Sidney Yates (D-Ill.) pleaded for the ban. Noting that his wife had been diagnosed as having cancer, Yates said:

"How does this happen? How can it happen? Where can it come from? . . . We are being subjected to so many cancer-producing influences in our society today, like toxaphene."