ARGENTINA AND BRAZIL are now holding talks to open each country's nuclear facilities to visits from the other. These reciprocal missions would be an important contribution to peace and to the control of nuclear weapons in this hemisphere.

Both countries have the capability to build nuclear weapons. There have been occasions over the years when each has seemed to be moving in that direction. Both have always denied it, asserting that they wanted nuclear power only for peaceful purposes. But because of a long rivalry between them, evidence of nuclear progress in either country has been grounds for anxiety in the other. Under the military government that collapsed in 1983, Argentina had been showing signs of moving purposefully toward a bomb. Both countries have declined to sgn the Nuclear Non-Proliferation Treaty, in protest against provisions that they consider discriminatory, and both have been carrying on nuclear work at sites that are not subject to international inspection under the treaty's safeguard system.

That's why it is particularly significant that the initiative here has come from Argentina, and that the visits would specifically include all of their nuclear sites without exception. Both governments emphasize that the visits would not replace other regional commitments, or the more formal safeguards that already apply to some sites in each country. But this agreement holds great promise for reassuring each government of the other's intentions. Perhaps it won't be limited to those two. The journal Nucleonics Week, which first reported these negotiations, says that Uruguay is also ready to join.

These talks would have been highly unlikely under Argentina's previous government, the junta. They are one of the many benefits that an elected president, Raul Alfonsin, is bringing to his country -- and not to his country alone. Progress toward the agreement has been delayed by the transition in Brazil, but there both the last government and the newly elected one have been firmly in favor of the idea.

It's a delicate business to fit together the network of treaties and understandings that try to prevent the spread of nuclear weapons. As long as a few countries have the bomb, and most do not, a kind of inequality is inevitably built into the general treaties. That inequality, reserving the weapons for the few, offends a number of governments, including some that have no intention of building them. Where those governments decline to join the non- proliferation treaty, regional agreements can complement it and support its purpose most usefully. When Argentina and Brazil begin sending their specialists to take a look at each other's nuclear plants, they will strengthen their own security, their neighbors', and everybody else's.