The Reagan administration repeated its charges yesterday that the Soviet Union has waged chemical and toxin warfare in Southeast Asia, saying a Canadian government study that provides evidence against the charge referred to "another time."

The Canadian data, released Wednesday after repeated inquiries from The Washington Post, showed that the poison being called a weapon by the United States is a naturally occurring poison that apparently is ingested with fungus-infested food.

The study took blood samples from 280 Thais in five areas of the country. It found that among 270 people in the sample who were not alleged victims of chemical or toxin warfare, five nevertheless had trichothecene poisons in their blood. Ten who were alleged victims of chemical warfare had none.

The United States has said trichothecene toxins have been used as a weapon and do not occur naturally in Southeast Asia.

The Canadian data shows that the toxins, which are produced by a fungus, do occur there naturally, and natural poisoning cases exist in a small percentage of the population, experts said in The Post's story yesterday.

But White House and State Department spokesman told reporters yesterday that according to the Post report, the Canadian study was based on work done in 1984.

"Our conclusion that chemical weapons use has occurred in Southeast Asia is based on evidence collected several years prior to 1984," said White House spokesman Edward P. Djerejian. "We believe, therefore, that the results of the Canadian study as reported in The Post do not pose a challenge to our earlier conclusions."

State Department spokesman Charles E. Redman said the administration stands by its judgment that "chemical and toxic weapons have been used in Southeast Asia and Afghanistan," although it had no recent evidence to confirm that such attacks were still being made.

The White House comments were challenged by researchers yesterday who said that the administration is getting bad advice about basic science. The Canadian report suggests there is a natural, background level of trichothecene poisons.

"The fact that the work may have been done in 1984 is not important. Whether it was done in '82 or '72 or '62 still gives us every reason to believe that trichothecenes occur naturally in Southeast Asia," said Bruce Jarvis of the University of Maryland.