IN A GRAND and sweeping gesture, the Regan administration proposes to end just about all its enormous farm subsidies -- if Europe and Japan do the same. The administration is showboating. The chance of actually getting a binding international agreement to do that, or anything remotely similar to it, is vanishingly small.

This kind of posturing is only likely to deflect the serious negotiations on farm subsidies that are getting under way. That's unfortunate, because all of the rich countries are spending far more than they want or need to spend on their farmers, and there's a useful deal to be struck.

Under the Reagan administration, the subsidies have more than doubled, mainly because of the overgenerous farm bill in 1985. In Europe, farm subsidies are pushing the Common Market into a financial crisis. Over the past several years, the United States and the Common Market have been throwing more and more money into their frantic efforts to offset and outbid each other's subsidies as they struggle to dump their farm surpluses onto the world markets. Together, the United States and the Common Market will spend $50 billion this year on farm subsidies.

But total abolition of subsidies is the wrong goal, and American attempts to press for it will only block progress toward the kind of money-saving bargain that is now possible. It's not only that, as everybody knows, farmers are voters who make political trouble for governments that forget them. It's also that many city dwellers are uneasy about their food supplies and want their governments to guarantee them. Japan is a densely inhabited island that remembers World War II, and Japan's governments will continue to support rice at whatever cost is necessary to ensure that at least a survival ration is grown at home. West Germany, currently blocking the attempt to rationalize the Common Market's crazy agriculture program, is a country in which people were literally starving 40 years ago. Things have got badly out of hand there, as they have here, but not many countries will choose to leave their basic food supply to the vagaries of world markets.

But how about an agreement to cut out all the export subsidies? They represent true waste. Along with them, how about the indirect export subsidies that are responsible for the surpluses that can't be sold except by dumping them abroad? Subsidizing farmers' incomes is a useful social policy, but the export subsidies are turning into an expensive scandal worldwide. As government here and abroad struggle to cope with their soaring costs, there's a chance at last to bring them under control. It should not be lost in a campaign to eliminate all farm supports -- an idea that is neither realistic nor desirable