The tea has been drunk, the smiles and little bows exchanged, and now the euphoria from Japanese Prime Minister Yasuhiro Nakasone's three-day visit to Washington has been replaced by a time of stocktaking.

What emerges here is a feeling that the new Japanese leader won a better understanding of his own internal political problems from the Reagan administration and on Capitol Hill, but failed to erase a deeply held feeling of skepticism about Japanese promises to ease their trade restrictions and open more of their markets to American goods.

"I think the trip helped in terms of improving the atmosphere, but it hasn't changed the fundamentals," said U.S. Trade Representative William E. Brock.

These "fundamentals" are serious: growing trade tensions that threaten the relationship between the United States and its closest Pacific ally. These tensions, which have led to mounting protectionist pressures on Congress from both labor and industry, are fueled by the prospect of a growing trade deficit with Japan this year. It may be erased in 1984 provided the American dollar weakens, making U.S. goods less expensive for foreigners to buy, and Tokyo opens more markets to U.S. products.

Underlying this all is the attitude of many Americans--including some in the Reagan administration and many congressmen--that the Japanese do not play fair on trade, that they bow and say all the right things, but in the end follow a "buy Japan" code and conspire to undermine industries of competing nations.

"There is a very strong conviction," said Brock, "that Japanese trading practices certainly are not acceptable and there is not equity in the trading relationship."

The extent of this feeling came through vividly to Japanese diplomats a few days before the Nakasone visit, as they listened to Larry King's late-night national radio call-in program while waiting for details of their government's new trade liberalization package to come clattering over the telex from Tokyo. One diplomat described as "shocking" the stream of anti-Japanese invective that flowed over the airways.

It is unclear how far Nakasone was able to defuse that feeling, though congressional leaders who met with him felt he had scored some points there with his politician-to-politician, straight-from-the-shoulder manner. House Majority Whip Thomas S. Foley (D-Wash.) called Nakasone more direct than other Japanese leaders he had met, while Sen. John C. Danforth (R-Mo.), chairman of the Senate Finance Committee's trade subcommittee, said, "Gosh, he's a good politician."

Brock also praised Nakasone's promise of "fundamental and important" reforms in what most American businessmen consider the unduly complex and restrictive testing and certification procedures for goods imported into Japan. Brock said these reforms, part of the trade liberalization program announced in advance of Nakasone's trip, could affect more than 60 percent of American imports.

The question remains, however, whether they will in fact ease the entry of American goods into what are seen as Japan's closed markets.

Brock recalled during a White House breakfast meeting with reporters Friday, the day after Nakasone returned to Tokyo, that past promises have failed to result in the sale of more American goods in Japan.

"We really have had an awful lot of talk," Brock told the reporters, "and not much change in substance." An agreement with Japan's telecommunications monopoly was reached during the Carter administration, for instance, "and we have yet to see that quantified into any increased trade for the United States," he said.

Beyond that, Brock mused over what must be a recurring nightmare for him: that their deeply held feelings of "insecurity and insularity" could lead the Japanese people "to believe it is unpatriotic to import things from other countries."

"What happens if and when the Japanese government does everything that we've asked them to do and things don't change that much?" asked Brock. "We have seen both former prime minister Zenko Suzuki and Nakasone attempt to address that public problem by telling the people of Japan it is in their interests to import just as it is to export."

The American trade official said he believes Japanese political leaders are convinced of that, but "a lot depends on the attitudes of the Japanese business community just as much as it does on the Japanese bureaucracy. They have to decide what in fact is their long-term national interest."

For the United States' part, Brock declared, "We are not going to allow them to continue to export if they don't import."

But that still leaves vexing trade matters hanging in the air.

It is clear from the Nakasone visit that there will be no quick action on the largely symbolic question of easing, or even ending, Japanese quotas on American beef and oranges. The Japanese prime minister was sent on his way to Washington with a demonstration from Japan's politically powerful farmers warning him against any concessions on those products, which means Tokyo is unlikely to take any action until after elections in August.

Nakasone, however, appears to have eased pressure on his government to make concessions in that area. One senior House member who asked that his name not be used suggested that American farmers were making too big a deal over beef and citrus, considering that Japan is the United States' biggest customer for farm products, buying $6.6 billion worth in 1981.

"The issue has been blown a little bit out of perspective, in all honesty," commented Brock. "It's important because it has taken on a certain symbolic significance," but the dollar amounts are so small they make little difference in redressing America's trade imbalance with Japan.

The question of continuing limits on Japanese car imports to the United States for a third and possibly a fourth year was barely touched on during Nakasone's talks here, but will be negotiated when Brock visits Tokyo next month.

Brock said the Americans warned Nakasone that the Japanese face the possibility of his office handing down unpleasant decisions for them on two trade complaints brought by U.S. companies--one on steel, the other on machine tools. But he said the Japanese should not regard any adverse decision as a sign that these talks had failed since the complaints are based on a pre-Nakasone historical record.

Brock called both cases uniquely different from previous trade complaints, and said his office is taking a serious, hard look at them. They both charge the Japanese with illegal trading practices that hurt American industries.

One of the minor flaps of the trip arose over one item in the trade liberalization package that preceded Nakasone here--a cut in tariffs on American cigarettes. It appeared that the Japanese tobacco monopoly had countered that move by raising its fees for handling foreign cigarettes, effectively nullifying the tariff cut. When American trade officials complained, Nakasone said he had ordered the Japanese company not to raise its fees.

"We did receive assurances," said Brock, "that steps would not be taken to offset the impact of the tariff cut."

Left out of the discussions, Brock acknowledged, is the Japanese practice that many believe is key to their control of overseas markets: the targeting of certain industries by the government, which provides protection from imports and research and development support as well as making heavy purchases itself.

"This amounts to an attack on certain U.S. industries," said Brock. "The problem remains as far as we are concerned."

Some in U.S. trade circles believe the Japanese are perfectly content to have the Reagan administration focus on beef and oranges, tobacco and such other fringe areas that barely affect the U.S.-Japanese trade picture as long as Tokyo is free to continue its policy of targeting specific industries for export growth.

Japan now is aiming at high technology--the computers, software and other sophisticated electronics that form one of the few industries where America maintains its competitive edge. There are growing fears that that edge can be lost to the Japanese if the government's targeting practices are not stopped.