PRESIDENT REAGAN keeps talking about the complexities of dealing with the Kremlin, and he is getting better at it. His latest effort, in California, drew criticism in some quarters, American and Soviet, for failing to reflect in full the somewhat upbeat mood in Soviet-American relations. But that is precisely why it was a good speech. The current mood, whatever it is, is never a solid foundation on which to build a sensible view of the Soviet Union. A historical dimension is essential, and that is what Mr. Reagan provided.

''Yalta'' gnaws at Ronald Reagan; he harked back to that wartime meeting of Roosevelt, Churchill and Stalin in his California speech. But how can Yalta -- symbol of the Soviet Union's broken promise of freedom to Eastern Europe -- not fail to weigh on any serious person considering how a democratic system and a totalitarian system are to get along?

Mr. Reagan paired allusions to Yalta with appeals for deeper cooperation between the great powers. There is a contradiction here, but a politically and morally unavoidable one, one that is at the permanent heart of international life and one that need not be crudely asserted but should not be prettied up either. Mr. Reagan's ''commitment to public candor about the nature of totalitarian rule'' sometimes offends Soviet spokesmen. But no one should suppose they are delicate blossoms that will wilt at the mere mention of a true word.

The Gorbachev policy of glasnost -- the selective opening up of Soviet society -- has portended not only a new character of life at home for Soviet citizens. It has also become the Kremlin's leading political export, a commodity that Moscow hopes to exchange for deference and good will. Mr. Reagan called it ''interesting,'' a description some found insufficiently cordial and enthusiastic. But what the president is saying is that Moscow should make its international reputation the old fashioned way: it should earn it.

In California, for instance, he suggested that the Kremlin publish its military budget, now a deep dark secret. A Soviet official visiting this country responded by saying that another Soviet official had indicated in a speech at the United Nations this week that the Kremlin was willing to do just that. But the lengthy official Tass account of the speech indicates nothing like that at all. It will be interesting to see when glasnost reaches the Soviet Union's military spending. Meanwhile, Ronald Reagan's query is right on the mark.