THE AMERICAN trade deficit is widening again, the January figures show. Last year's deficit was a record by a wide margin, with imports running $123 billion ahead of exports. This year, with the dollar's exchange rate higher than ever, it will probably be even larger. Trade deficits on this scale are unprecedented in economic history, and no one really knows what the full effects will be. But important changes in the traditional patterns of American trade are becoming visible.

It's not only that the volume of imports is larger than it's ever been. The composition of those imports is also quite different from past years'. Until a couple of years ago, the United States imported chiefly consumer goods and oil. It exported capital goods -- mostly industrial and business equipment, usually at the high end of the technology ladder -- and farm products.

But within the past two years, the United States has swung from an exporter of capital goods to an importer. Still more startling, the imported equipment increasingly comes from the most advanced areas of technology. Stephen S. Roach, an economist for Morgan Stanley, the investment banking firm, points out that last summer, the latest three months for which detailed figures are available, the United States imported $69 billion worth of capital goods (excluding cars and trucks) -- and nearly two- thirds of it was classified as high technology. What was it specifically? Computers, office machinery, communications equipment and instruments.

A tremendous boom of private business investment in computing and communications equipment began about a decade ago, and, with the high dollar, the proportion of this equipment supplied from abroad has been rising dramatically. Does it mean that American producers in these crucial fields are becoming less competitive? Not necessarily. Much of this imported gear is being produced abroad by American companies, with American technology and management but foreign labor. The United States, in the geographical sense, is no longer as dominant a source of supply for advanced electronics as it was earlier in this decade. But the American companies that make these products are another matter. The trade numbers alone don't tell you much about their standing, for the companies in those fields are now operating with little regard for national boundaries.

The huge trade deficit means that the country is living well -- for the present -- on a flood of goods for which it has not paid. It's a pleasant time for consumers. But the wave of imports is changing the structure of American industry -- and the impact is no longer limited to the aging factories that plod along with obsolescent technology.