IN THE WORLD'S foreign exchange markets, the price of the Japanese yen is now hovering around 190 to the dollar. That's a figure loaded with political significance. At 190 to the dollar, the yen is no longer demonstrably overvalued in terms of the goods that it buys and sells. As recently as September, it was trading at 240 to the dollar. At that price -- with the dollar much too high and the yen much too low -- the exchange rate powerfully distorted trade. It gave, in effect, a 26 percent subsidy to all Japanese goods sold here and imposed the equivalent of a tax of similar size on all American goods sold there. That was one reason for the gigantic American trade deficit last year. With the yen at its present level, that harsh subsidy-and-tax effect has disappeared.

The dramatic and rapid rise of the yen over the past five months seems to have been largely the result of two political decisions, one made in Washington and the other in Tokyo. The Reagan administration, reversing its position of the previous four years, decided to intervene forcefully in the market to push the dollar down. The Japanese government, anxious about rising American resentment of Japanese imports, decided that it was time to let the yen rise.

The speed and precision of the operation has been a genuine surprise. The wash of hundreds of billions of dollars through the exchange market every day has greatly diminished the control that governments used to exercise over their currencies. Many people -- including ourselves, in this space -- had argued that it was no longer possible for governments to set exchange rates by purely financial tactics. But here the world's two leading financial powers seem to have done just that.

The next question is whether they can reliably hold the rate there. If they do, that success will give momentum to the Reagan administration's cautious approach toward much more active international management of the exchange rate system. The dollar remains significantly overvalued against most of the European currencies.

But there is a danger here for the United States. Americans aren't in the habit of saving much of their incomes, and as long as their federal government runs a deficit of $200 billion a year, or even next year's Gramm-Rudman-Hollings target of $144 billion, it will depend on a heavy flow of foreign capital to finance that rising debt. The overvaluation of the dollar is the result of that inflow of foreign investment, bidding up its price against other currencies. That's why Americans have to bring their budget deficit down before they can make much more progress toward stable exchange rates -- and rates that do not discriminate against American trade.