IT IS ALWAYS good to hear Fidel Castro call for improved relations with the United States, as he did in his fascinating interview with The Post published yesterday. But considering the historical record, not to speak of Mr. Castro's fine print, the prospect doesn't seem so promising.

The record shows that for at least 10 years Cuba and the United States have been discussing the worthy but modest items -- immigration, travel, hijacking, coast guard, fishing, radio stations -- on their bilateral agenda. Progress on particular items is occasionally made: just last December the Reagan administration, abandoning its refusal to talk with Cuba, made its first agreement with Havana, on immigration. But the items that remain to be discussed are, as Mr. Castro says, less important. Long ago the two countries found they could live with ragged bilateral ties. Full agreeement on all the items extant would not materially alter the basic hostility.

Mr. Castro says he sees no sign of a basic American policy change. Mercifully, there has been a change since the shameful episode in which official U.S. efforts at sabotage and assassination were undertaken. These have been exposed and repudiated. But in another sense, Mr. Castro is right. No single American regional policy has shown more consistency longer than the general American outlook on Cuba. John F. Kennedy defined it in 1963, saying no real progress was possible until Cuba changed its relationship with the Soviet Union and its support of revolution. Mr. Castro has now reaffirmed that these fundamental elements of his policy are still in place. He has always been ready to "normalize" relations -- by which he means to accept the end of the American trade embargo and other forms of American pressure. But, as he emphasized again to The Post, he has never been ready to improve relations at the expense of his general foreign policies or, as he puts it, his "moral principles."

This was not Mr. Castro's only reference to morality in the Post interview. He cited, at length, his moral values in volunteering a rejection of a charge in a Post editorial of Dec. 17, 1984, that he had cynically dumped mental patients and criminals on the United States in the 1980 Mariel boatlift. One can understand why Mr. Castro would like to erase that sequence from his and others' minds. Permit us to restate the record:

Three Cubans, desperate to leave the island paradise, had broken through police lines into the Peruvian Embassy compound. Spitefully, Mr. Castro removed the police and challenged what he still chooses to call "anti-social elements" or "lumpens" to get out. Some 10,000 Cubans flooded the compound -- to go not to the golden United States but to poor Peru. An embarrassed Mr. Castro, hunting for a way to identify the departures with Cuba's traditional foe, opened the port of Mariel for a boatlift to Miami. Among the 125,000 who left were inserted several thousand criminals and mental patients, their status compellingly established by their own and their boatmates' testimony. It was cynical of Mr. Castro to dump them then, and it is cynical of him to deny it now.