PRESIDENT CARTER took up last week the other menace to arms control - the one that the United States creates when it ships weapons into, let us say, the Persian Gulf. Governments have always recognized the extreme danger in the nuclear balance among the superpowers. But until this year Americans behaved as though their vigorous salesmanship of such old-hat weaponry as combat aircraft, missiles and tanks were merely normal commerce. Mr. Carter, to his great credit, has changed that.

From now on, he said, this country will regard arms sales as an "exceptional" instrument of foreign policy. In the past, these sales went forward routinely unless they positively damaged American interests. Now the reverse is to be the rule: The sales won't go forward without evidence that they positively contribute to our national interest.

The President's statement doesn't end the issue. It's only the beginning. A large part of this country's enormous arms exports to customers like our NATO allies, who buy cautiously. Some go to Israel, under longstanding commitments. The trouble arises with countries like Iran and Saudi Arabia.They are rivals, their oil revenues bring them great wealth, and neither country has enough trained people to operate the highly sophisticated military equipment that they are buying.

President Nixon laid down a series of bad precedents five years ago when he began using large-scale shipments of these weapons to butter up the oil states. It started with the Shah of Iran. It got worse two years later, when the end of the Arab oil embargo was closely followed by arms contracts between the Saudis and the United States. The Nixon and Ford administration tried to justify this by citing the importance of the oil.

But the importance of the oil is precisely the reason for starting to scale down the arms traffic.It is creating a massive concentration of extremely powerful armaments in an underdeveloped region, on the stability of which - by bad luck - the prosperity of most of the world now depends. Mr. Carter will be pressed by a variety of sheikhs and monarchs, but above all the Iranians, to keep demonstrating his regard for them, and his usefulness to them, by continuing to deliver the supersonic fighters, the guided rockets, the radars and armored cars. It's not always going to be easy to say no.

Mr. Carter has set out a number of exceptions in his statement and left himself plenty of room to maneuver. There won't be any immediate cutoff; the Carter administration has already approved nearly $5 billion worth of new arms sales. But the President has firmly committed himself to a reduction in the sales, and more attention to their cumulative effects. In the past, these shipments have rolled mindlessly forward, accompanied by official assurances that each case was being judged on its own merits. Unfortunately the past five years' Persian Gulf sales, taken together, are piling up more than enough tinder to ignite one gigantic conflagration.