With the arrival of the New Year, three decades have now passed since Fidel Castro took over as supreme ruler of Cuba, declared the Western Hemisphere's first Marxist-Leninist regime, and started 30 years of roller coaster-style relations with the United States.

As Castro heads into his fourth decade as the island's dictator, there are signs that he may be mellowing a bit. Although still preaching and actively exporting revolution, Castro is no longer entirely the young, fire-breathing radical he once was. He may even be looking for status among world leaders as a kind of elder statesman, a status that precludes the radical revolutionary reputation he has groomed since 1959.Over the years, relations between Cuba and the U.S., only 90 miles away, have run the gamut from comic opera buffoonery to tragedy. At various times, Americans have thought of Castro as, alternatively, the clown prince of the Caribbean, a ruthless Latin American dictator with his boot heel on the collective necks of his people, and a stooge of the Soviets.

The tragic part of the Castro era can be measured in human lives: The U.S. invasion of Grenada that was needed to crush a Cuban-supported Marxist regime there; Cuba's adventurism in Angola, totaling more than 300,000 troops over the last decade; and the thousands fleeing Cuba in the Mariel boatlift eight years ago.

In the past year Castro has made some encouraging concessions, agreeing to a peace settlement in Angola that includes the pullout of Cuban troops. He has released thousands of political prisoners from Cuban jails, responding to international charges of human rights violations. And, it appears that immigration and visitation privileges could be restored, reuniting many families split between the U.S. and Cuba.

In some of these moves, Castro may have little choice. Surrogate Soviet support for international military adventures and regional conflicts is waning. Castro's domestic economy is faltering, as Cubans wonder when the labor of 30 years of building a revolution is going to start paying off with consumer goods. The stream of dead, wounded, and AIDS-stricken Cuban soldiers returning from Africa is difficult to conceal in a small, island nation.

Maybe, just maybe, Castro is beginning to confront something no rhetoric, no amount of Soviet support, and no legions of troops can conquer: his own mortality and, with it, his place in history.

All these signs point to possibilities of better relations between the U.S. and Cuba. President-elect Bush, the eighth U.S. chief executive to deal with the Cuban problem, is said to be considering the appointment of lawyer William D. Rogers as chief policymaker on Cuba. Rogers held that post in 1975 under President Ford and led a brief effort to normalize relations with Cuba, a move sunk by the two nations taking up arms on opposite sides in the Angolan civil war.

The eight years of the Reagan Administration have been marked by a staunchly conservative policy of opposition to and isolation of Cuba. It was the same attitude the administration at first adopted toward the Soviet Union.

Just as U.S.-Soviet relations have mellowed, the U.S. can reasonably consider the possibility of relaxing its policy toward Cuba along the same lines, at least to the extent of opening discussion on matters of mutual interest.