Two Cuban crocodiles stretch lazily in simulated sunlight at Washington's National Zoo, having recently crossed the considerable geopolitical and ideological gulf between Fidel Castro's Cuba and President Reagan's America.

The distinctively patterned, black-and-white-skinned reptiles came as a gift to the zoo from Cuban scientists. Washington zoo officials said the crocodiles represent the maturing of a scientific relationship that has grown despite chilly relations between the United States and the communist Cuban government.

Much like the zoo's giant pandas, a gift from China in 1972 that helped symbolize a turning point in Sino-American relations following President Nixon's visit there, some Smithsonian Institution scientists view the gift of the crocodiles as the start of a new phase of scientific cooperation between Cuba and the United States.

The crocodiles, which weigh about 100 pounds each, are not currently on display. They probably will continue to live in special feeding tanks in the basement of the zoo's reptile house until they are large enough to fend for themselves among the zoo's larger crocodiles, zoo officials said. Fully grown, Cuban crocodiles may weigh as much as 600 pounds and measure about 12 feet long.

The newly arrived rare Cuban crocodiles join four other adult Cuban crocodiles and seven newly hatched ones.

Ross Simons, program manager of the Smithsonian's office of the assistant secretary for science, said of the U.S.-Cuban scientific exchange program now under way: "We have been hoping for that kind of exchange ever since the first delegation went to Cuba in 1978."

Simons was among a group of U.S. scientists who went to Havana in 1978 to help establish an exchange program with the Cubans. The visit eventually led to the gift of the two freshwater crocodiles to the United States this fall.

A State Department official said the exchange program, called the Smithsonian-Cuban Academy of Sciences Scholarly Exchange, has had no "significant" impact on U.S.-Cuban relations. But in terms of impact in the scientific world, Simons said, the program has opened doors for the western scientific community that had been slammed shut following the communist takeover in Cuba.

"Scientific interests override any political considerations," said Simons, a bearded man with an easy manner. "If you approach these exchanges in an apolitical framework, they tend to work in part because it is scientist to scientist."

Through the program, about 20 U.S. and Cuban scientists have exchanged visits, studying plant and animal life through a range of specialized disciplines. This year, for example, Lourdes Rodriquez Sehettino, a expert on reptiles from Cuba's Instituto de Zoologica, studied Smithsonian collections for three weeks, learning new techniques for keeping lizards alive in laboratory conditions, according to zoo officials.

Some U.S. scientists have returned from Cuba claiming to have gained, besides scientific data, a better understanding of the people and their island country.

"I have a lot of friends down there," said Dale L. Marcellini, curator of the National Zoo's reptile house who has traveled twice to Cuba to study its lizards, some unique to Cuba. "I certainly understand Cuba a hell of a lot better. And I'm sure they understand us a whole lot better."

The exchange program had its origins in a brief letter from former National Zoo director Theodore H. Reed to Abelardo Moreno, who directs Havana's zoo, at an international meeting of zoo directors in 1977. In the letter, Simons recalled, Reed noted that Cuban and American scientists had a commonality of scientific interests on which a working relationship could be built.

"It turns out that Moreno was the perfect person to contact because he had worked at the Smithsonian in the 1940s," Simons said.

In spring 1978, a delegation of 10 Smithsonian scientists took a chartered DC-3 from Fort Lauderdale, Fla., to Havana -- because no commercial airline flew there at the time. They toured scientific facilities and began to work out an agreement. In return, a Cuban delegation visted Washington and New York City, Simons said.

In 1980, then-secretary of the Smithsonian S. Dillon Ripley flew to Havana and signed a memorandum of understanding, formally marking the start of the exchange.

The results, said Simons, have been important. After visiting Cuba, a diverse island laboratory that is key to understanding the region's current and past ecological systems, scientists such as Porter M. Kier have published major works.

Kier, a Smithsonian paleobiologist who is one of the world's leading authorities on small crustaceans, published in March the voluminous "Fossil Spatangoid Echinoids of Cuba," based primarily on his ability to study a Cuban collection of sea urchin fossils that was feared lost after Fidel Castro's rise to power in 1959.

After the takeover, Kier said, he was unable to correspond with Mario Sanchez Roig, a Cuban physician who spent his life assembling an extensive array of fossilized sea urchins.

Kier said he feared that the collection would be broken up and lost. For more than 20 years, no Western scientists had heard of its whereabouts.

"I assumed I would never see the material," Kier said. "It was quite a disappointment."

But through the exchange program, Kier traveled to Cuba and discovered that the fossil collection had been meticulously cared for and stored.

"I was delighted," he said. "Through the [Cuban] Academy of Sciences, I went back to Havana to bring the collection back." Kier also was able to see the places where Roig discovered the fossils, which is important to his work. The collection eventually was returned to Cuba.

"We have enjoyed relations in an apolitical way with scientific communities throughout the world," Simons said. "We will oftentimes work in countries that the United States does not have diplomatic relations in."

"There is," he continued, "an interrelatedness to the world, and animals are not bound by political boundaries."