President Reagan tried a carrot-and-stick approach with five tiny east Caribbean countries today, coupling a promise of U.S. economic aid with a warning about the dangers of following the Cuban and Soviet economic and political system.

Reagan bluntly spelled out the conditions of U.S. assistance in after-lunch remarks to leaders of the five island nations.

"El Salvador isn't the only country that's being threatened with Marxism, and I think all of us are concerned with the overturn of Westminster parliamentary democracy in Grenada," Reagan said. "That country now bears the Soviet and Cuban trademark, which means it will attempt to spread the virus among its neighbors."

The president's comments continued the harsh anti-Cuban line he has taken during his five-day self-styled "working holiday" in the Caribbean. On Wednesday night in Jamaica he said that the Cuban experiment in Marxism, "a philosophy alien to this hemisphere," had meant "economic deprivation and political repression" for the Cuban people.

White House officials said today that Reagan's comments on Grenada were the strongest he has ever made about the left-leaning government in that country which came to power in a March 13, 1979, coup and has since forged close political and economic links with Cuba.

Reagan said he wasn't automatically excluding Grenada from his Caribbean Basin initiative, which will distribute $350 million in economic aid and eliminate U.S. tariffs on most of the region's projects. But he also declared that there would have to be "visible indications of change" in Grenada's relationship with Cuba if the island government expects U.S. aid.

Afterward, Barbados Prime Minister G. M. G. (Tom) Adams made a comment that showed that he is less concerned than Reagan about the Grenadan example. Adams said he "hadn't had the occasion to think about Grenada for six months until the press questioned me about it."

However, Adams praised the U.S. Caribbean initiative, saying that his country will "benefit substantially" from elimination of U.S. tariffs on Barbadan rum and other products.

According to a senior Reagan administration official, leaders of the other four countries were less sure that they would be helped by the Reagan plan.

The other participants in today's meeting were the prime ministers of Antigua and Barbuda, Dominica, St. Kitts-Nevis and St. Vincent and the Grenadines. These four nations, with a total population of slightly more than 300,000, are so small and underdeveloped that they lack the roads, electricity and water to encourage the private development which is the cornerstone of the Reagan plan for the region.

Administration officials said Reagan understood this view despite his own preference for private rather than government investment.

"I think the president learned a good deal about the region today," one official said afterward.

If so, it was the last learning experience of this kind that Reagan is likely to have during the remainder of his visit here. An official reception tonight was the last public event on Reagan's schedule except for a five-minute radio broadcast he will make Saturday.

The president is scheduled to divide his time here between the island's sunny white beaches and a villa owned by an old Hollywood friend, actress Claudette Colbert.

Originally, the Reagans were supposed to stay at the Colbert residence but security considerations prompted the White House instead to accept the hospitality of Fort Worth furniture manufacturer Paul Brandt, who owns a luxurious home on a secluded golf course a couple of miles away.

Reagan arrived in Barbados today to a military welcome that included an unintentionally spectacular "19-gun" salute. Since the Barbados military forces have no cannon, the ceremony was conducted by ground explosives. One of the rounds ignited a grass fire at the airport which enveloped Air Force One in smoke but was quickly extinguished by a water truck.