ANTITRUST POLICY struggles constantly with a paradox: Competition is crucial to healthy markets, but a certain kind of competition in which the biggest fish swallow the little ones will end in no competition at all. The people who set the antitrust rules have the job of deciding who cannot safely swallow whom. That's why the Federal Trade Commission has voted to go into court to oppose a massive consolidation in the soft-drink industry.

Coca-Cola proposes to buy Dr. Pepper, and until they heard from the FTC, PepsiCo had intended to buy Seven-Up. That would have left the industry's two largest companies with more than 80 percent of the market. If the FTC had let these two acquisitions pass, it might just as well have shut the door, turned out the lights, and gone out of the antitrust business altogether. When two strong companies hold four-fifths of a national market, the prospects are not brilliant for their smaller competitors' trying to stay alive in whatever nooks and crannies the giants leave them. At first glance it might seem easy for new competitors to get into such a simple business as putting soda into bottles and selling it. In reality, the business (like most businesses) is anything but simple, and its network of bottling franchises and marketing arrangements makes it a difficult market to break into.

The remarkable thing about this pair of sales is that they seemed worth trying. These experienced and well-run companies thought that they just might be allowed to go through with them. Coca-Cola and Dr. Pepper's owners still think so, and will fight the FTC in court. That in itself is a measure of the extent to which the Reagan administration has changed the regulatory atmosphere in which mergers and acquisitions take place. The old rules have been relaxed. Coca-Cola is now pressing the question whether they have relaxed far enough to allow one of the dominant companies in a large industry to buy a significant competitor. The definitive answer will have to come from the courts. But for business competition in general, it will be a great deal better if that answer is no.

These cases illustrate some of the limitations of the continuing campaign for deregulation. The idea is to get the regulators out of the way, with all their constraining rules and finger-wagging, and leave the game to the pure forces of competition. It's a fine idea up to a point, but the soft-drink cases show you pretty clearly where that point lies. Competition works well until the big winners begin to buy up the other players. Then the regulators and the courts have to step in.