The United States and Japan, Prime Minister Zenko Suzuki reminded his audience, are two elephants confined in the same enclosure.

"Combined, we have one-third of the world's total gross national product," he said through an interpreter. Collisions between the two economic super-powers are both enevitable and dangerous, he added, and it is the task of both countries' political leaders to control these encounters, he told a group of Washington Post editors and reporters.

And that is why he is proud of Japan's decision last week to hold back its shipments of passenger cars to the United States during 1981 and 1982 -- an arrangement that only a politician could love.

U.S. auto executives say the Japanese action will do them little, if any, good. Although the Japanese government has told its auto firms to trim auto shipments to the United States to a total of 1.68 million in the year ahead, a 7 percent reduction from the past year, the figure doesn't include the 400,000 Japanese cars already in this country, awaiting sale.

The decision was needed to prevent Congress from clamping limits on Japanese auto imports, said General Motors Corp. president F. James McDonald this week. "But I don't think it will have any effect on the marketplace, as far as we're concerned," he said.

If the auto policy arranged by Suzuki's government and the Reagan administration is no answer to the U.S. industry's prayers, it may be even more unpopular in Japan, Suzuki indicated. The prevalent opinion is that the U.S. auto companies have themselves to blame for their economic plight, he said, because they didn't move quickly enough to produce smaller, fuel-efficient cars. The mood of the Japanese is, "don't blame me," he said.

But smugness is an unaffordable luxury where U.S.-Japanese trade issues are concerned, the Japanese leader said. Convinced that Congress would impose trade restrictions on Japan if the auto issue wasn't settled voluntarily, Suzuki acted, fearing that other countries might follow the U.S. example, setting off a chain reaction of economic nationalism.

The trade tensions between the two countries don't end with automobiles, he continued. "It is inevitable that from time to time in certain sectors, friction will arise. I want to emphasize that it is vitally important to deal with these sources of friction in an early stage," he added.

The expanding field of electronics and communications equipment, mentioned several times by Suzuki, will provide a hard test of how far Japan is willing to curb its own self-interest to maintain its trading relationship with the United States.

Japan's central telephone company, Nippon Telephone and Telegraph, has agreed under pressure from the United States to give U.S. firms a chance to compete for NTT's $3.3 billion in annual purchases of communications equipment. "People often say the Japanese market is closed," Suzuki said. "It is a thing of the past. It is now a myth."

But U.S. firms must learn how to sell in Japan and how to tailor their products for the Japanese market, and must try harder, he said. "We welcome them with open arms," he added.

That assurance is still received skeptically by U.S. companies.

"The fact is that even with all the prostestations of good faith, their market is closed and ours is open," said John Sodolski, vice president of the Electronic Industries Association. "They can sell into a world market from a secure domestic base. That gives them a tremendous advantage," Sodolski said. It also "generates a lot of bad feeling" among U.S. firms that face competition from Japanese manufacturers at home in the $20 billion U.S. market as well as abroad, he added.

Japan translates its technical journals into English, has sponsored English-language briefings on contracting procedures for U.S. firms and has invited a high-level U.S. communications trade group to Japan this summer for further talks.

But, says Sodolski, "We still don't understand how they do business, who the key people are, where the decision points are."

The issue will remain on the two nation's political agenda well into the future.