The first Commerce Department official to make an official visit to Cuba in 17 years predicted that if the political problems between the two countries could be resolved, U.S. companies' exports to Cuba could reach $390 million by 1981.

Lawrence Theriot, who has followed Cuban trade since 1974, visited Cuba during the first week of February. He said he interviewed 22 officials in five days, including Foreign Trade Minister Marcelo Fernandez Font.

All trade with Cuba was banned in the early 1960s as a reaction to Cuba's seizure of American businesses, but some aspects of the embargo have been eased in recent years. Theriot estimated that representatives of 400 U.S. companies have visited Cuba recently.

Although the potential Cuban market is only a "drop in the bucket" in terms of total U.S. trade, it could be very important to companies in certain sectors, Theriot said.

The Cubans are particularly interested in agribusiness, and hungry for all kinds of products and know-how in that area, he said.

Theriot added that what he called "general logistics" is a major problem in Cuba, saying Cubans need the means for "moving people and goods - everything from forklifts to buses."

Cubans also have expressed interest in American manufactured goods that they consider of superior quality such as medical and data processing equipment.

In his report, published in the department's biweekly "Commerce America," Theriot pointed out that most of Cuba's trade is conducted on a barter basis with other members of the Communist trade bloc Comecon.

About 30 percent of Cuban trade is with capitalist countries, he said, and the main suppliers are Canada, Spain, Japan and West Germany. This percentage can be expected to increase during years in which Cuba has more hard currency, such as 1975 when the world price for sugar was high and 48 percent of Cuba's imports were capitalist countries.

Theriot cautioned that some of the businessmen who have visited Cuba are overly optimistic about an early resumption of trade. He pointed out that the process of normalization of relations is stalled, primarily because of U.S. objections to Cuban military activities in Africa.

If trade were to resume, the Cubans would expect to earn as much in exports to the United States as they spent on imports, Theriot said.

He said the major potential exports are sugar, nickel, lobster,shrimp naphtha and tobacco, although tariffs on cigars would be very high.

Theriot estimated that sugar exports could begin at 500,000 tons a year and eventually grow to one million tons.

Tourism is becoming another important source of hard currency for Cuba, he reported. Twenty-five hotels are being built and the old ones - many built by American companies before the Cuban revolution - are being refurbished.

He estimated that Cuba may earn $110 million in hard currency from tourists in 1981. The earnings for 1977 were only $25 million.

Theriot, who stressed that the report reflected his personal views. said visiting Cuba could be useful to some U.S. businessmen for planning. Because Cuba has a centrally planned economy, "The general guidelines and goals of economic development and trade are already set through 1980 for some industries," he said.

Because the staff of the Foreign Trade Ministry has been very stable since Fidel Castro took power in 1959, contacts made now may prove useful in the future, he said.

For some American companies, Ciba's centrally planned economy "permits virtually one-stop selling to a developing market of 9.5 million people," Theriot said.

Theriot said his three-hour interview with Foreign Trade Minister Fernandez was particularly valuable bacause the minister went over the figures in a report Theriot has prepared on Cuban foreign trade. The Commerce Department plans to publish the report next month.