The mammoth electronics producer Toshiba Corp. has told employes to turn off office lights during the lunch break. C. Itoh & Co., one of Japan's big trading houses, is dragging its feet in signing new contracts. Hiyashi Ceramics Inc. is preparing to petition the government for financial aid.

The strong dollar that helped power Japan's export surge of the past 1 1/2 years is gone for the time being. Its departure has had only minor effect to date on balance sheets here, but corporate managers are planning seriously for the possibility it will not be returning soon.

Japan has had four bouts with a cheap dollar since the world shifted to a floating-rate currency system in 1973. Through each it has managed to prosper, albeit with some dislocations, and there is little reason to expect that things will be different for the fifth.

The dollar's latest decline began in late September, after the so-called Group of Five -- the United States, Japan, Britain, West Germany and France -- agreed to drive it down with coordinated intervention in foreign currency markets. The dollar was buying about 240 yen when the campaign began; this week, it got only around 205.

A low dollar means that Japanese products tend to become more expensive for Americans and American products cheaper for Japanese. And U.S. officials hope this will rein in a galloping trade imbalance between the two countries, which may reach $50 billion in Japan's favor this year.

To do that, however, the dollar must stay low. Whether it will is anybody's guess. The five governments have vowed to keep up pressure in the markets, but skeptics say long-term changes will require cuts in what are seen as fundamental reasons for the strong dollar: U.S. interest rates and the federal government deficit.

Even if the U.S. currency does stay low, economists here predict, impact will come slowly. "The effects will not be seen in this fiscal year," which ends March 31, predicts Kyoji Kitamura, a deputy vice minister at the Ministry of Finance. "They will begin in the following one."

The rate of growth in the trade imbalance might slow in the next fiscal year, but according to the Japan Economic Journal, the country's premier financial newspaper, tentative calculations by "highly placed monetary sources" show that if the dollar stabilizes at 210 yen, an actual fall in the surplus will not be seen until the year beginning April 1987.

Delays occur because goods can take six months or more to flow through the pipeline of international trade. Videocassette recorders ordered when old rates were in effect may not reach their destination until well into next year. And once Japanese manufacturers raise their prices to compensate for the new exchange rates, to the extent they do, the market will need time to react and start buying less.

Kitamura says the government's "highest priority" is to keep the yen strong. But this effort toward international good will has plunged companies here into a bewildering new financial maze.

"We are changing all our assumptions," said a top manager at C. Itoh and Co., where negotiations for many purchase and sale contracts have been frozen pending stabilization of the exchange markets.

Major exporters were not caught cold when the dollar began its plunge, however. Some, like C. Itoh, claim to have turned some short-term profits with shrewd dealing as it slid. Longer-term protection has come through a skill learned years ago, smoothing out currency rate peaks and valleys with forward contracts from foreign exchange markets.

Sony Corp., for instance, which makes about 70 percent of its sales abroad, is covered for about half of its dollar earnings at an average rate of around 240. That means that regardless of what a dollar is fetching on the day-to-day market, Sony until next spring will get 240 yen for half of its incoming dollars.

Impact has come much more swiftly to smaller export industries, where financial planning tends to be less sophisticated and the pipeline faster flowing. Japan's porcelain industry, which has exports of around $1 billion a year, for instance, is badly squeezed already, says Akira Nishimura, managing director of the Japan Pottery Manufacturers Federation. "Buyers are coming to Japan, but they won't sign contracts," he said.

By some accounts, even before the fall, Japanese chinaware prices were about 25 percent higher than other Asian export countries'. But higher quality kept the orders coming in. Now, the price is 40 percent higher, and buyers are scared off.

Another group already feeling hurt are foreign tourists. Overnight, Japan has gone from being just expensive to being very expensive. Hotel rooms cost 15 percent more than they did two months ago. "Those who pay us by cash often express surprise at the rate," said a spokesman for Tokyo's New Otani Hotel.

Takashi Ishihara, chairman of the Japan Automobile Manufacturers Association, has been quoted as saying that a 220-yen dollar is the break-even point for the export industry as a whole. A 200-yen dollar, over the long term, he said, would be "a devastating blow."

Each companies' vulnerability is different. In the coming months, as protection by currency futures tapers off, Japanese exporters will face tough choices. "In January or February, we'll decide whether to raise prices," said Sony Managing Director Tsunehiko Ishizuka. "It's too early now."

Sony Corp. of America has already announced that it will raise prices on its entire line of consumer, industrial and professional electronics products by 5 percent to 12 percent, but, for the present, the export prices at which it buys from Sony in Japan have not changed.

Profit margins are generally low in Japan's electronics industry. It does not have an option open to companies with high margins -- holding prices and swallowing losses by cutting profits. The industry where that would be most likely would be the Japanese auto industry, whose earnings in the United States have swollen because of high prices brought on by export quotas.

Cutting costs is a third option open to everyone. The low dollar, in fact, could inject new efficiency into industries that have grown flabby on easy profits overseas. Auto manufacturing executives, in particular, believe that the quick money made in the United States has precipitated a dangerous fall in competitive verve.

"We are asking our factories to cope with the possibility of a 210-yen dollar in the coming year," said Sony's Ishizuka. Toshiba reports it is trying to save $25 million by March 31 on business travel, office supplies and communications and other savings.

"We think mental pressure is important," said a Toshiba spokesman. "So we're telling people, let's turn off the lights at lunch when we're not working."

Small industries, meanwhile, are looking to the government for aid. Porcelain makers have already called at the Ministry of International Trade & Industry and offices of the ruling Liberal Democratic Party to plead for interest-free loans to tide them over.

The dollar's plunge also holds enormous potential to effect capital outflow from Japan. Life insurance companies and other major institutional investors here have plowed billions of dollars into U.S. government securities in recent years, because the yields are higher than on Japanese paper.

Each investment dollar for which they paid 240 yen a few months ago will now bring only 205 if they cash it in. Much of the loss is only on paper, however, as Japanese companies traditionally invest for the long term, not for speculation. Many will hold on, hoping for rates to improve.

Net outflows were already on the way down when the dollar began falling. They decreased from $8.8 billion in July to $6.5 billion in August and $3.1 billion in September. Much of that was attributed to narrowing of interest rate differences. Figures for October, during which the dollar was down, are not yet available.

Japanese newspapers reported that foreign money surged into the Tokyo Stock Exchange in the week following the dollar's decline, 40 percent more than the previous week, as Japanese securities overnight became a cheaper buy for foreigners.

And some Japanese companies with dollar-denominated debt, meanwhile, began paying some of it off in advance to take advantage of the rates while they lasted.

The sagging dollar, of course, is good news for those who handle Japan's approximately $140 billion a year in imports. Oil and many of the natural resources that drive Japanese industry, in fact, have been twice cheapened, by declining world prices and by the declining dollar. That could be an important boost for Japanese industry as a whole.

For the country's nine power-generating companies, for instance, a one-yen decline of the dollar's value for one year translates into a $20 million saving on oil bills. When the dollar declined in the late 1970s, they came out about $2 billion ahead of projections for one fiscal year and users got a small price reduction.

Foreign consumer goods should become cheaper and help a buy-foreign campaign that Prime Minister Yasuhiro Nakasone is pushing in an attempt to cut the trade deficit. But major department stores in Tokyo note that what is on the shelves now was ordered months ago at the old exchange rates. "At the moment, we have no schedule to change prices," said a spokesman for the Mitsukoshi department store chain.

Last week, the government's Economic Planning Agency announced it would pressure the retail industry to pass savings on to the consumer. But, noting that power and gas companies are heavily saddled with new investment costs, officials indicated cuts there were unlikely.

But in fact, there are a few places in Japan where the cheap dollar is already bringing low prices. Restaurants at the Prince Hotel chain are a case in point: they offer a special $10 steak, for which the customer pays in yen based on the day's exchange quotation. A chain spokesman reports it has become an especially hot item.