THE DISCUSSIONS between General Motors and Toyota regarding joint production of a small car in this country are not a reassuring development. On either side there is a hint that the company is making a gesture to pacify criticism--but doesn't want to invest much money in it for the long haul. GM evidently thinks that it needs a new model to succeed its Chevette in the subcompact range, but doesn't want to go to the considerable expense of developing it. Toyota seems to think that it needs to produce some cars in the United States to meet the protectionists' outcries, but prefers to avoid building plants of its own. But those are the affairs of the two companies, and their shareholders. For the rest of the world, the issue here is competition.

GM is the largest automobile company in the world by quite a wide margin. Toyota is third. (Second, if you were wondering, is Ford.) One joint operation, particularly on the limited scale apparently anticipated here, hardly constitutes a merger. Nor is the worldwide automobile industry at present highly concentrated. But this kind of an entente between No. 1 and No. 3 raises an obvious question.

After all, GM can't have it both ways. The American antitrust tradition would disapprove of GM's rising share of the American market for domestic cars. But the American antitrust tradition is increasingly obsolete. For a rapidly expanding list of products, the share of the domestic market is not what counts. In a lot of businesses, certainly including the automobile business, it's the company's share of the world market that is crucial. Although GM makes nearly two-thirds of the American cars sold in the United States, that market remains highly competitive because of the imports. But the same logic makes you wonder about the future of that market if the largest domestic manufacturer and the largest importer begin to work together.

There are now about two dozen automobile producers of real international importance. Present evidence suggests that by 1990 the number will be down to 10 or so. If two of the most aggressive and, in financial terms, strongest have meanwhile developed a pattern of joining forces, that does not look like a formula for a highly competitive automobile market.

There is little in the antitrust laws of this country, or any other, that addresses worldwide competition. But that is the scale on which the management of any large and well-run manufacturing company is now thinking. The GM-Toyota talks raise a question whether governments as well might not have to give it some thought.