A recently completed set of U.S.-Japanese trade talks broke down barriers to sales of American-made drugs and medical equipment and should provide manufactures with greater opportunities to sell their products in Japan, Treasury Assistant Secretary David C. Mulford said yesterday.

U.S. manufacturers currently hold less than a $1 billion share of Japan's $25 billion market for drugs and sophisticated medical equipment, the second largest in the world.

"There's an enormous market potential out there. Since the reforms will help expand the role of imports, U.S. industry should benefit considerably," Mulford said in a press conference detailing the results of the talks in the health sector.

The talks were part of a larger series of trade negotiations set in motion 13 months ago by President Reagan and Prime Minister Yasuhiro Nakasone. Talks in two of the sectors -- heath and telecommunications -- were declared a success last month by Secretary of State George P. Shultz and Foreign Minister Shintaro Abe, while they said more work is needed in two other sectors, electronics and forest products.

In detailing the results of the health talks, Mulford said they "helped open the market, and now it's up to U.S. industry to increase the sales."

Frank Samuel, president of the Health Industry Manufacturers Association, said the agreement appears to address some long-standing problems facing the medical-products industry in Japan and should give it a chance "to turn around our growing trade deficit with Japan in medical products."

But he cautioned that past agreements with the Japanese have not produced the intended results.

Recognizing that, Mulford said "the real test will come in the implementation of these measures," and added that followup sessons will be held every six months, at least through this year, to make sure the arrangement is working.

A joint report that he and Japanese Vice Minister of Health Hitoshi Yoshimura released yesterday provides the framework for implementing the agreement, he said. Mulford, assistant Treasury secretary for international affairs, was put in charge of the heath-care-area talks, which were part of the Market Oriented Sector Selective (MOSS) process supervised by Shultz and Abe.

In New York, meanwhile, American and Japanese toy makers signed a three-year agreement to increase U.S. exports to Japan. Currently, U.S. manufacturers ship about $6 million worth of games and educational and pre-school toys to Japan. Japan sends more than 20 times as much to the United States -- $140 million worth of toys such as GoBots and Transformers.

Under the agreement, Japanese companies will increase the amount of their imports by $2.8 million this year, $5.6 million in 1987 and $8.8 million in 1988.

But the U.S. manufacturers had to agree that none of the made-in-America toys is the kind that compete directly with Japanese-made products.