MARK DOWN the Madrid conference on European detente as the occasion for Ronald Reagan's first formal agreement with the Soviet Union and Yuri Andropov's first with the United States. Against the tension otherwise clouding Soviet-American relations, this demonstration of a capacity to come to terms is the most important result of the negotiations on human rights, cooperation and security that the two countries have been conducting, with 30- odd European states, for almost three years.

The agreement has not actually been signed but there seems no doubt that it will be soon. Moscow is not in the habit of pulling the rug out from under its own negotiators. Mr. Reagan has done it only where the negotiators were under his predecessor's instructions.

Credit is due to the negotiator, Max Kampelman, originally Jimmy Carter's appointee. What made it possible for him to continue effectively from one administration to the next was the emphasis of each on keeping in step with the United States' allies and friends. Washington accepted Madrid as a forum devoted first to building Atlantic unity, and it deferred to the Europeans' obvious special interest in broadening European d,etente.

The agreement keeps alive and vigorous the "Helsinki process" begun in the 1970s. Moscow accepted it essentially to win favor for its political objectives in Europe, and the West accepted it to make life in Europe -- both parts--more humane, regular and safe. The process committed the signers to a list of obligations and, for enforcement, to a continuing series of conferences at which delivery on those obligations is reviewed.

The Madrid conference reviewed Soviet performance in Poland and Afghanistan and on the matter of human rights in general. It tightened marginally the language of the obligations to which the signers will henceforth be held. It set up a schedule of eight future sessions, some on the expert level, some on the political. One of these, starting in Stockholm next January, will give the Kremlin some chance to make propaganda hay on disarmament. But the session is to address first "confidence-building measures" (advance notice of military maneuvers, for instance) meant to ease the West's fear of a Soviet surprise attack.

It is thankless gritty work to push Moscow to observe human-rights commitments it adopted in much cynicism. But it would be an unconscionable denial of its basic values if the West, having once taken up the task, put it down.