Mexico has become the largest exporter to the United States of heroin and perhaps of marijuana, according to an annual State Department report on worldwide drug production.

The report by the State Department's Bureau of International Narcotics Matters called Mexico's drug-eradication efforts the "principal disappointment" of 1985. It blamed "drug-related corruption" in Mexico and increased activities by traffickers.

Mexico's "once-effective opium poppy and marijuana eradication program has become less effective," it said. "The indication at year's end was that Mexico was perhaps once again the leading exporter of marijuana to the United States, and the single largest source of heroin."

It said diminishing government control over drug production was "in part a result of concerted efforts by traffickers to reestablish cultivation sources in Mexico and in part because of an apparent spread of drug-related corruption which has affected every facet of the enforcement program."

It said the kidnaping and murder last year of U.S. drug enforcement agent Enrique Camarena in Mexico highlighted the corruption.

The report, delivered to Congress yesterday, is produced annually by the State Department as an estimate of worldwide narcotics production as well as cooperation of other countries in shutting down the flow of illegal drugs.

Mexico is expected to produce 21 to 45 metric tons of opium in 1986, the same as last year, compared with 21 tons in 1984 and 17 tons in 1983, the State Department found.

Mexican marijuana production is expected to be as much as 3,000 tons again this year, as in 1985 and 1984. In 1983, before the major eradication efforts, it was 4,975 tons.

The report said Colombia, "once viewed as one of the most difficult countries in narcotics control," destroyed 85 percent of the marijuana grown in its northern coastal area, a major growing site. It praised Colombia, Jamaica and Belize for their combined eradication of more than 4,400 tons of marijuana during 1985. Fourteen important drug-producing nations are actively destroying drug crops, compared with two countries four years ago, the report said.

Federal law prohibits continuation of foreign aid to countries judged by the president to be making inadequate progress toward stopping narcotics production. But no drug-related cutoff has ever been instituted, and there is no mention of such a possibility in the latest report.

"The fact remains that worldwide production of illicit opium, coca leaf and cannabis marijuana in 1985 was still many times the amount currently consumed by drug abusers" in the United States and elsewhere, it said.