NEVER MIND ALL that business about man's last best hope and the rest - the U.N. General Assembly is a tough audience for anyone who is serious about wanting to put some restraints on armaments and the technologies that sustain and spread them. The buyers and sellers were all there yesterday, when Mr. Carter addressed the United Nations: the nuclear haves, have-nots, wish-to-be's and maybe's. His remarks were concentrated on the urgency of controlling the great arms competition and reducing present arsenals. We thought his speech was eloquent and strong and that it stressed the right things.

The President's optimistic appraisal of the protion that this country would even be willing to reduce its nuclear weapons by half "on a reciprocal basis" and his statement on the use of nuclear weapons were attention-getters. But his discussion of related issues was more audacious, considering the nature of the audience. To talk about nuclear nonproliferation, the curbing of conventional arms sales and a comprehensive ban on nuclear testing is to challenge the strategies and ambitions of a very large part of the U.N. membership. Mr. Carter was distinctive and clear on the subject of so-called "peaceful" nuclear explosions: "My country believes that the time has come to end all explosions of nuclear devices, no matter what their claimed justification. . . ." Only a few weeks ago, in connection with international fears that South Africa was on the verge of exploding a nuclear device, the French foreign minister made the same point bluntly - i.e., that there is no such thing as a peaceful explosion.That this view is gaining credence and being explicitly made these days is a very promising development.

It was, however, on the subject of curbing the spread of nuclear weapons that Mr. Carter spoke with the greatest intensity. He addressed some of the cynicism and confusion that surround the subject and took the trouble to try to explain why he thought it was not too late to control the dangerous technologies or to prevent the proliferation of nuclear arsenals. Mr. Carter was speaking to an audience composed mainly of representatives of developing nations, and many of them have objected strenuously to the efforts of a small group of industrialized nuclear-supplier countries to agree among themselves on a code of restraints that will make the commerce in nuclear energy materials less risky, less likely to give the buyers of such materials the option for a bomb as part of the bargain. Nonetheless, the President insisted that this suppliers group should continue to meet until it has completed its work and fulfilled what he regards as its obligation. He spoke out for "full-scope comprehensive safeguards," which implies a measure of control of nuclear plant and fuel that many importing countries do not think they should have to put up with. And once again he made his pitch for alternatives to the plutonium fuel cycle, "alternative fuel cycles that can be managed safely on a global basis."

To be sure - and fittingly, since the setting was the United Nations - Mr. Carter was dealing only in words here. There are surely some questions about the rate at which this administration is fulfilling its stated goals concerning curbs on U.S. sales of conventional armaments abroad, and also questions concerning the President's own fortitude on the proliferation issue. In parts of his administration in the past several months there had been an appreciable slackening of the antiproliferation effort. For that reason especially it was heartening that Mr. Carter spoke at such length and with such obvious conviction about his commitment to halt the spread of nuclear arsenals. We are thinking not just of his U.N. audience, but also of some of the recalcitrant folk in his own government, when we say there are times when speeches matter.