President Fidel Castro rejects the latest congressional overture toward lifting the U.S. trade embargo against Cuba, calls the Soviet Union the freest country in the world, says his government holds close to 3,000 political prisoners and meets Barbara Walters.

It all happens on the latest of Castro's appearences in the U.S. media, an hour-long ABC News program to be broadcast tonight.

But for all his evident desire to explain himself to U.S. audiences and the several steps toward better U.S. Cuban relations taken since the Carter administration took office, Castro tells Walters that he does not think normal relations can be established quickly. "Maybe in Carter's second term, between 1980 abd 1984," he said.

In a program that includes "pop" scenes like a motor launch crossing of the Bay of Pigs during which Walters asks whether Castro can shed any new light on the assassination of John F. Kennedy (he can't), Walters also elicits answers on subjects Castro usually doesn't discus.

For the first time in many years he puts a number to the total of political prisoners he holds.

There are 2,000 or 3,000 he said, down from more than 15,000 "when the activity of the U.S. was more intensive against Cuba." He ater indicated the number is close to 3,000.

The U.S. State Company recently estimated the total at Between 10,000 and 15,000.

Walters and Castro, speaking through an interpreter, argue over the Soviet Union and China.

Castro's statement: "I thing [Russia] is the freest of all countries," brings an amazed reaction from Walters and an argument over Soviet dissenters that ends with Castro saying:

"Why do I have to tolerate the allies of my adversary? If you want to tolerate them; O.K., but not me."

Castro refuses to call China either his freind or enemy, but says: "I consider China as a good ally of the United States."

Later he adds: "We have internationalist relations with the Soviet Union and China has reactionary relations with the United States." He alleges, despite plentiful evidence to the contrary, that China agrees with the United States on all fundamental issues.

On other subjects, Castro dismisses the Senate Foreign Relations Committee passed measuring that would permit Cuba to buy medicine, agricultural supplies and footstuffs from the United States but would ban such U.S. purchases from Cuba.

Under those terms, Castro said, "we would not buy anything at all in the U.S., not even an aspirin for headaches, and we have lots of headaches."

Castro says Cuba's relations with former President Ford and former Secretary of State Henry A. Kissinger were so bad that Cuba rooted for a Democratic victory last fall.

President Carter, the 50-year-old Cuban leaders said, is "an idealistic man with certain ethic principles."

Castro said he is convinced that under Carter there are no U.S. plots to assissinate him.

Castro gave Walters a message for Carter in in which he said: "I am honestly and seriously interested in improving relations and am going to think of what ways I can help."