The largest U.S. trade mission ever assembled to peddle American exports arrives here today to test the theory that Japan's markets can be opened up in the era of the sinking dollar and the rising yen.

The 141-member mission, composed mostly of representatives of lesser-known companies that have never seriously tried selling here, will be looking for markets ranging from fish to automobile seat covers.

Besides its obvious goal of sales and profits, the mission also will be testing both this country's long-standing claim that Americans can sell products in Japan only if they try hard enough and its more resent commitment to lowering its huge balance of payments surplus. So far, neither effort has shown much successes.

The semi-government Japan external Trade Organization (Jethro last week issued a report showing that since 1970 the U.S. share of the Japanese market has declined across the board.

It showed, for example, that the American share of Japan's machinery imports fell from 60 to 51 percent and that the U.S. proportion of both durable and non-durable consumer goods declined by about 14 percentage points.

During the same period, the report noted, European and developing countries increased their share of the Japanese market.

The trend showed that administrative barriers and a complex distribution system within Japan are not main obstacles to foreign traders, as Americans have persistently complained, the report commented. What is needed, it said, is a greater American effort to sell in Japan.

The Japanese view the large trade mission with a mixture of agreeableness, nervousness and skepticism. The skeptics says that it is merely another one-shot American probe of Japanese markets and will have little lasting effect.

One government official observed recently that none of the companies represented on the mission has bothered to print sales catalogues in the Japanese language. An American official confirmed that only one of the companies, Borg-Warner, has prepared a Japanese-language sales brochure for this trip.

The Japanese also are fearful that a failure to make immediate sales will disappoint the U.S. mission and revive claims that this country does not tolerate imports of manufactured goods.

"We fear there will be a counteract because they expect too much," one government source said, "and this will make the Americans angry."

Nevertheless, the government and private industry are putting out a huge welcome mat. A joint committee has gathered more than 100 staff members to show the Americans around and make appointments with potential customers. At latest count, they has set up more than 3,600 appointment.

Officials of both countries have warned against lofty expectations. An American official here emphasized that it is a long-range development mission to make initial contacts, not a "two-week blitz" for immediate comtracts.

The Japanese are being especially hospitable to the mission because of renewed worries about the effects of their huge trade surplus. Many believe that a new and more serious confrontation with the United States is just around the corner.

Japan had set a target for reducing its current accounts surplus to about $6 billion in the fiscal year that ends next March 31. It will actually be far highes - the government now expects it to reach nearly $14 billion. Even though Japan's exports to the United States are declining in volume, the surplus continues to rise in dollar terms, partly because Japanese firms raise prices in the United States to match the appreciation of the yen.

Japanese business circles were stunned recently by a blunt warning from U.S. ambassador Mike Mansfield. In a supposedly private meeting with the powerful Federation of Economic Organizations, Mansfield said that "the feeling of frustration over the U.S.-Japan trade imbalance is mounting further, within and outside the U.S. Congress."

One Japanese government official said he regards the next six months as the most perilous period because it will take that long before the surplus shown signs of declining in dollar terms.

It was originally hoped, he said, that imports of American goods would increase since the dollar's decline would make them cheaper for the Japanese. Instead, he said, the high rate of inflation of the United States, is raising prices and eroding that competitive edge.

The U.S. mission, which arrives today, will be led by Mark Shepherd Jr., chairman and chief executive officer of Texas Instruments, Inc., and will include 131 business executives and 10 staff members from the Department of Commerce. Commerce Secretary Juanita M. Kreps and Assistant Secretary Frank Weil will accompany them.

Few industrial giants are represented in the mission. Most are small-or medium-sized coorporations such as New Hampshire Trading Corp., Empire Tool Co., and Arion, Inc.

U.S. officials here said the purpose is to introduce newcomers to the Japanese market, help them to make contracts, and hope they will plan initial investments leading to long-term commitments. Most of the major American corporations maintain offices and staffs in this country.