Leading Japanese economists and monetary specialists predict an exchange rate of between 90 and 110 yen to the U.S. dollar by 1990 or sooner under current conditions.

The general reason given for these forecasts, beyond an anticipated continued weakness of the dollar's value, is the greater adaptability to prevailing business circumstances of Japanese corporations compared with their U.S. and European counterparts.Japanese companies have been far more successful than expected in cutting back their production costs in order to deal with the steady climb in the value of the yen against the dollar, explained an economist at Long-Term Credit Bank of Japan in a typical statement.

These opinions emerged recently in a series of private lectures and seminars where the economists declined to be quoted by name by attending members of the press due to the sensitivities involved in predicting future exchange rate ranges of Japan's currency.

Most Japanese economists agree there is nothing on the horizon which should reverse the trend in the exchange rate situation. However, in almost all cases they emphasized they believed that even the present exchange rate in terms of the dollar produces an overvalued yen.

Where for many years we had an overvalued dollar and a badly undervalued yen, stressed one prominent Japanese economist, there is now a reversal of this position.

Some of the economists, however, expressed hopes that by the end of this decade the yen's exchange rate would be closer to 120 to the dollar than their statistics allow them to predict.

In almost all instances the economists said they would be surprised if there was any change in the concerted intervention of the U.S. and Japanese authorities to prevent the value of the dollar from plunging too sharply or too rapidly.

But there appeared to be general agreement that one of the difficulties the authorities in both countries are continuing to face is the belief in the market that intervention by the U.S. Federal Reserve doesn't appear to be sufficiently serious.

Whether this will improve over the next two years, said one monetary expert, clearly depends on the future mind-set of officials of the Fed toward the dollar. I'm not all that certain these people have yet reached a decision on the proper level of the U.S. currency.

The basic assumption of Japanese economists for the moment seems to be that the Fed should be much more forceful in establishing some sort of at least stated floor for the dollar's value.

Certainly right now it is difficult for anyone to point to a narrow range in the yen-dollar exchange rate and claim this is where the Fed stands and is willing to fight to preserve its position with the assistance of other central banks, commented one Tokyo monetary expert.

Without such a stance, in the opinion of a growing number of Japanese economists, there will be continuing fears in the United States and around the world that at any moment the international financial markets could once again be thrown into confusion.

What we need right now is some type of reassurance that the U.S. authorities have a solid hold on the situation and sufficient determination to do whatever is necessary, suggested one monetary analyst.