In normal times, the prime minister of Japan does not concern himself with how fast a computer transmits data over his country's telephone lines or who deserves a seat on a board that will certify new products for sale.

But in past months, Yasuhiro Nakasone has fretted over these and other esoteric questions. They have come to dominate critical negotiations with the United States over ways to increase foreign telecommunications sales in Japan.

At his official residence in Tokyo, Nakasone is briefed frequently on the talks, including the latest technical details. He has sometimes lectured his negotiators on the need to accommodate the Americans, Japanese officials say.

"The Japanese people may think we are giving too much favor to the United States," Nakasone told a budget panel of the Diet, or national legislature, yesterday, "but the state of the two countries' relations is very worrisome."

Emotions have been aroused in Washington by a trade deficit with Japan of close to $35 billion last year. The U.S. government has seized on telecommunications as a test of Japanese resolve to open its vast domestic markets in all fields and reduce the deficit.

Nakasone has promised action. Some believe his pledges grow from a profound vision for a new internationalist order in Japan. Others see him as a politician who is merely responding to pressure from abroad.

But by almost everyone's account, he is not getting the results he had hoped for. Many of his plans seem to have met staunch opposition from career bureaucrats who draft, debate and must ultimately approve every change made in the rules that govern the economy of Japan.

Like all Japanese prime ministers, Nakasone has a tenuous grip on office. Rival factions in the ruling Liberal Democratic Party continue to plot his removal and he may feel he will undermine his standing if he appears to be too deeply in the pocket of the Americans.

The telecommunications talks are a national news story here. When an American team is in town, television news broadcasts include footage of negotiators facing off across tables in Tokyo.

"For over one century, 100 years, Japanese have been indoctrinated to export or die," said former foreign minister Saburo Okita, now part of a private council seeking solutions to the trade dispute. They now are learning that imports are crucial to good relations with the rest of the world, he said.

But opinion polls show that many Japanese see no reason to abandon protectionism, which helped repair the devastation of World War II. In a June 1984 survey by the prime minister's office, 33 percent of those polled said Japan should refuse to open up, in order to protect its own industries. Forty-one percent favored opening to imports and the remainder were undecided or gave unclear replies.

Japan's domestic market for telecommunications goods and services was almost hermetically sealed until 1980. That year, Tokyo formally agreed to let foreign companies sell to the state-owned monopoly Nippon Telegraph and Telephone (NTT). Its procurements of more than $3 billion per year account for most of the industry's volume.

By 1983, U.S. companies' sales had grown to $131 million, still only a fraction of what Washington maintains they could manage if the market were truly open. Japan's decision to denationalize NTT effective April 1 and open the field to competition led the United States to push for more changes starting on that day.

From the start, the Japanese have maintained that they will establish a system that favors neither domestic nor foreign companies. But the United States, citing issues like data transmission speed and membership on certification boards, contends that many barriers are being kept in place.

When American technicians come to Tokyo to negotiate issues like these, they generally sit down not with politicians who have promised progress, but with civil servants who may have worked with the issues at hand for their entire careers.

The prime minister appoints only two, and in some cases, three officials to each ministry. Careerists oversee everything else. Respected for technical knowledge, they have traditionally played a much greater role in policy-making than do their American counterparts.

Each ministry has a cadre of "elite" bureaucrats. Recruited from the best universities, they begin service knowing they will rise to the top. When serious concerns are at hand, they work around the clock, napping on cots and subsisting on instant noodles.

Many believe they know better than any politician what is good for Japan in their particular field and they may resist change if the famous Japanese consensus has not been reached. Foreigners citing the promises of the politicians may be rebuffed.

"We've had comments like, 'That's fine for a vice minister to say, but vice ministers come and go. We don't,' " said one U.S. official.

Intricacies of the faction system inside the ruling Liberal Democrats and old rivalries between Japanese ministries seem to have figured in the talks' pace, too.

Americans say the most dogged resistance to change has come from the Ministry of Posts and Telecommunications. It is traditionally controlled by the faction of former prime minister Kakuei Tanaka, who remains a "shadow shogun" in Japanese politics despite a conviction on bribery charges.

Its stubbornness must be approved by Tanaka, who is held to exercise special influence all over Japan, many Americans believe. Nakasone depends on Tanaka's support to remain in office. Therefore, it is assumed, Nakasone's power to protest is restricted.

That ministry, traditionally a minor leaguer in bureaucratic politics here, has seen its standing rise due to the talks. In a few cases, the Americans have been backed up by their old nemesis, the Ministry of International Trade and Industry, which has often battled with the smaller ministry over regulatory turf.

Japanese officials argue they have made key concessions in past months. Some U.S. negotiators see the exercise as another rout for their side. Perhaps due to U.S. perceptions of lack of progress in telecommunications, Nakasone is giving ground in two of three other areas the two countries are discussing -- electronics, forestry products and medical products.

Earlier this week, the government announced it would submit legislation specifying that computer software qualifies for 50-year protection under Japanese copyright law. The United States welcomed the step.

The government also suggested it might cut tariffs on forest products, another U.S. demand. Earlier, it had ruled this politically impossible due to financial troubles in Japan's domestic industry. But this week, the ruling party said it may increase aid to the industry and that this would make tariff cuts feasible.

Officially, the two sides are not discussing auto exports, but they remain at the back of everyone's mind. Four-year-old quotas on exports expire at the end of this month and the government is calling for moderation from manufacturers. Many analysts think it may impose informal quotas if a surge of exports threatens to sour relations with the United States even more.

Nakasone apparently believes steps like these are a small price to pay for the bounty Japan enjoys from the United States. In addition to military protection, the United States is its most important foreign market.

In 1984, the Japanese economy turned in its best performance in five years, fueled to a large degree by a flurry of export orders from the United States.

American officials concede that even if all trade barriers into Japan came down, only about a third of the deficit would disappear. The Japanese say the share would be smaller, but either way, Japan would remain well in the black