Japan is on the brink of its most serious economic slowdown since the 1973 oil shock, some economists here believe.

With the newly strengthened yen starting to cut into exports and buyers at home unable to pick up the slack, some estimates project that the rate of economic growth will slow to as low as 2 percent in the coming year, about half the rate of the fiscal year that just ended.

Not everyone shares this assessment -- the government is officially predicting 4 percent growth. But enough people do so that sober analysis is going on in corporate offices and economic journals here. Occasionally, the word "emergency" is heard.

In the rest of the industrialized world, it would be called a routine dip in the business cycle. Unemployment is unlikely to move much above 3 percent. Inflation will be close to zero, and modernization of factories and equipment will continue.

Still, it would mark an upset in an economy founded on predictability and possibly portend major changes in how this nation's 120 million people earn livings. It also could give foreign companies a chance to retake some of the ground lost to Japan in the past decade.

In some eyes, the predicted slowdown is an inevitable and ultimately positive development that will close out an obsolete postwar development strategy drawing expansionary steam from exports, and bring in a new, more mature one that stresses the buying power of the country's citizens.

It would be a mistake to assume these new challenges will get the better of Japan. It took it only a year to start the bounce-back from the 1973 oil shock. New efficiencies being forced on its industry will serve it all the better when happier times return.

For the time being, however, the country's exporters are getting scared. "We have cut costs and waste," said a spokesman for Toshiba Corp., one of Japan's major electronics producers. "But there is a limit to what we can do."

Japan's economy actually shrank a bit in the year after the 1973 oil crisis. But since then, it has cruised along enviably with a growth range of between 3 percent and 5 percent, even through the turbulence of the Iranian revolution's second oil shock in 1979. Growth was about 5 percent in 1984 before falling to about 4 percent in 1985.

In the private sector, almost no one is taking seriously the government's prediction that the 4 percent can be maintained in the 12 months from this April 1 to March 31, 1987, Japan's new fiscal year. Nomura Research Institute is betting on an expansion of about 3.3 percent. Sumitomo Bank and other mainstream financial institutions say it could be as low as 2 percent.

Such projections start with the value of the yen. Last September, fearing a backlash of trade barriers abroad if its balance-of-payment surpluses continued to balloon, Japan joined the United States, Britain, France and West Germany in a program to lower the value of the dollar.

To the surprise of many, it worked extremely well: The yen has hit record highs against the dollar and now is trading at about 175 to a dollar, 35 percent stronger than in early September.

The logic was that a weaker dollar and stronger yen would help reduce the trade surpluses by making Japanese automobiles, videocasette recorders and other popular export items more costly to foreign buyers, and foreign products cheaper to Japanese.

Japanese export earnings would be sacrificed, in part, to avoid something worse -- protectionism.

Officials here are chagrined that exports as reported in dollars have continued to rise since September, because the dollar value of everything shipped abroad is now 35 percent higher. But those figures will change later this year, they say; already the number of units exported is down.

Some Japanese see a silver lining in the situation. They feel their vaunted export companies have grown soft with the high dollar, making sales regardless of effort. Now factory pep talks and in-house newsletters are ordering employes to cut waste and raise productivity, reviving the old spirit of the post-war recovery. Executive travel, perks, even office lighting also have come under scrutiny.

But many companies also have gone reluctantly for price increases overseas. Auto makers have raised theirs by close to 10 percent since last fall. Electronics prices are up, too. The financial press here reports plans for rises by makers of sewing machines, food-processing equipment and construction vehicles.

Damage to the economy as a whole so far has been minor, however. Big companies have been cushioned by cash reserves, forward currency contracts and the fact that foreigners still buy Japanese if the price goes up, because they often think the quality is superior. Sales of Japanese cars, for instance, soared by 10 percent in January over the previous year, even though the cars were costlier.

Worst hit are small and medium-size enterprises, which lack the management sophistication and financial reserves of their larger counterparts. Chinaware companies, for example, have felt the pinch, and they have trooped into politicians' offices in search of financial aid. The government now is offering low-interest loans.

A heavy impact on exports is expected to begin appearing around the middle of 1986. It will push down the economy as a whole as big companies cut orders from subcontractors, subcontractors cut orders from suppliers, and so on. Japan's preference for "sourcing" most manufactured components at home will turn against it in this case, concentrating the negative effects within the country.

Other symptoms of decay also are being noted. A Nomura study found profitability among manufacturing companies down by 31 percent in the past six months compared with the same period in 1984-85. Investment in new plants and equipment, another barometer of business confidence, also has dropped since the year's start.

Much of the damage to exports, however, will be offset by help on the import side. Despite an image as a monolithic exporter, Japan depends on imports (it bought about $118 billion worth in 1985) because it has few resources beyond its people's diligence. Just about anything made here starts with something bought abroad.

Japanese companies now find that many of the imports, denominated in dollars, cost 35 percent less in yen terms. With oil, which accounts for about 30 percent of all imports, the gain is even more startling, because dollar-quoted prices for it have been falling rapidly, too. Each dollar drop in the price of a barrel of oil saves Japan about $1.2 billion a year.

Japan's big electric power companies are reaping windfall profits through lower fuel bills. Some of the savings will be returned to consumers, but much of it is expected to go into major modernization spending for the plants, giving a boost to stagnating capital investment.

The dollar value of imports is expected to decline faster than that of exports during the coming year, meaning Japan's overall surpluses will widen. Sumitomo Bank's chief economist, Masahiko Koido, predicts the surplus is going to hit $60 billion in the coming year, up from about $50 billion in the past year. Declines in the surplus might be expected in the following year, analysts say.

Despite the lower cost of imports, the net effect of the currency realignments will be negative, in most analysts' eyes. If the country is to hold its momentum, it must find another source of growth. And most eyes turn to government for this.

"It must turn the economic trend upwards," Koido said. "This would lead to more imports and a better balance-of-payments picture."

In his national budget for the fiscal year starting April 1, Prime Minister Yasuhiro Nakasone has included what his officials call a potent program for increased domestic demand. It provides for increased capital spending and encourages local governments to issue bonds to improve roads, sewers and public works projects on which Japan traditionally has shortchanged itself in favor of commercial and factory development. It has tax and other incentives for private companies to start housing and other big-ticket items.

But general expenditures, which most analysts say offer the greatest potential for pump-priming, will provide little impetus. That is because the Japanese government finances about 20 percent of its spending through bonds (proportionately more than the United States), and Nakasone has taken steps to make budget-balancing a theme of his 3 1/2-year-old administration. The country's great surpluses are in private hands; Japanese are as reluctant as any people to turn over money to the government.

With economic pressure mounting, the government has shown some flexibility, however. "The present goal is that, by 1990, the issue of new deficit bonds should go to zero," said Masanori Morioka, a director of C. Itoh and Co., one of Japan's leading trading houses. "But the emergency economic situation may lead to this being postponed by two or three years."

Big gains in consumer spending seem unlikely. The current hard times for corporate profits will make it difficult for Japanese workers to reap significant gains in their annual "spring offensive." Even if they did, it is unlikely much of the money would reach cash registers of shops and department stores.

Japanese tend to be proficient savers, salting away 18 percent of earnings in 1984. This was considered a virtue in the early postwar days but, in this decade, foreign and Japanese leaders alike blame it for worsening Japan's chronic poor showing on imports.

Nakasone has twisted corporate arms to increase imports, but his televised appeals last year to the Japanese people to think foreign when shopping has had little effect. Most people here still seem to think that the best of most things are made at home.

In another effort towards stimulation, the government this spring has twice cut the discount rate its central bank charges to commercial bank borrowers, shaving a point to put it at 4 percent. There is talk -- and hope among business leaders -- of a third cut, but no promise yet from the government.

Many economists say the government's fundamental problem is that it wants things both ways. It wants to hold on to a strongly deflationary fiscal policy and, at the same time, stoke the economy to keep the standard of living improving and draw in more imports. Something will have to give.