The dollar's exchange rate has been declining, at a dignified and tolerable pace, for about five months. So far it's down a little more than 10 percent from the peak early last March. That means it still needs to come down another 25 percent or so to reach its actual value in terms of the goods that Americans export and import.

If it comes down too slowly, the overpriced dollar will continue to generate tremendous trade deficits and, in Congress, protectionist legislation. If it comes down too fast it will create a surge of inflation, as imports suddenly become more expensive; that would be followed by sharply rising interest rates. The past half-year's decline seems to have been at just about the right pace -- fast enough to show visible improvement but without malign side effects. The question is whether it's going to keep going that way.

Exchange rates are now being set in the minute- to-minute trading among banks and brokers that deal in foreign currencies. Since last winter, foreigners have become a little less wildly enthusiastic about holding dollars. One prominent reason is that interest rates in this country have been falling, making investment here slightly less inviting. Another is that these foreign investors already hold enormous numbers of dollars, and their eagerness to keep adding to those holdings at last year's rate seems to have weakened marginally. That slight cooling of their ardor is faithfully reflected in the daily movements of the exchange rates.

Americans need about $2 billion a week in credit from foreigners to finance their trade deficit. The foreign moneybags are making Americans pay a little more for their money as the dollar keeps declining, a little at a time, from week to week. It's all been very orderly and serene. And it's moving in the right direction. So why worry?

Because the next step in this process is always utterly unpredictable. If the American economy begins to grow faster this autumn, as the Reagan administration expects, corporate profits will improve and interest rates will rise. That could reverse the trend in the foreign exchange markets, sending the dollar higher, foreshadowing still wider trade deficits and still more vehement political reactions against imports. That is the nature of the dilemma in which this country now finds itself.

The real point of vulnerability is that $2 billion a week of foreign lending that the United States must have, regardless of cost, to finance its trade deficit. As long as the United States needs that money, its economy will operate subject to terms set by the perceptions and prejudices of foreign investors. The United States is now a debtor nation, and debtors have to realize that their debts always mean a certain unwelcome loss of control over their affairs.