It could reasonably be said that Fidel Castro's revolution ruined a large part of Fernando Mena's life.

Although he fought for the revolution in the late 1950s, Mena ended up turning against it. In 1962, he was charged with counterrevolutionary activity and began a six-year prison term.

Since his release, Mena, now 51, has lived in Miami, where he works for Seven-Eleven Stores. With prison and exile, he has spent only 11 of 27 years of marriage living with his wife, who was not permitted to leave Cuba.

Yet, Saturday night, Mena and his wife sat drinking Castro's liquor, watching the floor show at a government-paid visit to the Tropicana night club. Early that morning he had joined scores of other exiles in applauding and shouting "Bravo" to a Castro speech. Then Castro stood silently with his head bowed as the exiles had serenaded him with the Cuban national anthem.

Castro's sweeping prisoner release and family reunification program, signed Friday night with a group of 140 exile representatives, may not have changed the basic disapproval many of them feel for his government.

But for many in the exile group that came here, and for those in the United States they represent, it has meant the reawakening of senses long dulled by hatred and an apparent willingness to accept Cuba as it is today.

For Castro, aside from the humanitarian aspects of the program, it has turned out to be a political and public relations coup of significant proportions.

Perhaps the most telling incident reflecting a new exile attitude came during Castro's post-midnight new conference after the signing. Asked about his relations with the United States, Castro vehemently denounced, as he has for years, the U.S. economic embargo that is the communist government's symbol of "capitalist aggression" against Cuba.

His voice rising, Castro called "the Yankee blockade" a "weapon of pressure.. .a indecent thing." He was interrupted by applause and cheers from the exiles a number of whom in the past have protested loudly at any U.S. concessions to Cuba.

Obviously, not all Cuban exiles in the United States-estimated at 600,000-feel the same way. Some of those who have participated in the dialogue with Cuba since it began several months ago report death threats and business boycotts. The exile media in Miami, they say, has largely refused to cover their activities.

But according to Mena, who said he is 'neither Castroite nor Communist," but a "social Christian-with the emphasis on Christian"-95 percent of the exile community supports them. Others put the percentage lower, but still in the majority.

Even if they have not become firm supporters of the Cuban revolution, one of the visiting exiles marveled, "Castro made us do something the Revolution considers revolutionary. And he made us feel good about it."

In addition to treating the exiles to a VIP charter airplane, whose two-hour flight here from Atlanta on Dec. 7 quickly became a floating lounge with free-flowing Havana Club rum and the thick smoke of Cuban cigars, Castro publicly massaged their egos.

At the Saturday morning news conference, he repeatedly referred to the exiles, formerly known as "worms" in local Cuban slang, as "the true representatives" of the "Cuban community abroad."

He praised their "courage and valor in defying the threats...of terrorist elements in the United States opposed to the dialogue."

Some of Cuba's 3,600 political prisoners, particularly those who have refused to take part in rehabilitation programs here because they have not wanted to legitimize the government, have denounced the program as a political ploy.

"They thought that one day there would be a great invasion from the United States," Castro said, "but it never came."

Noting that he did not want any "voluntary prisoners," Castro said they would be released "no thanks to the United States, but to the government of Cuba," which he said was doing its "revolutionary duty" in reuniting Cuban families, "and to this dialogue."

For many in the exile group, it was the separation from friends and families that for years fueled their active opposition to the Cuban government. More than any change in politics, it is the reunification with friends and relatives that appears to have tempered them.

Fernando Mena's wife is to leave for the United States this week.

Salvador Madruga, a Miami businessman who was part of the Bay of Pigs invasion force, obtained the release of his ailing father from prison last week. For participation in the dialogue, he said, he has been expelled from the militant Miami Bay of Pigs veterans organization.

Rigoberto Herrera, 23, a Hyattsville, Md., resident who left Cuba for the United States with his parents in 1970, was tearfully reunited with his elderly grandmother, one of the hundreds who stood with their noses pressed to airport terminal windows as the charter arrived.

When a tape of Castro's news conference was broadcast, crowds of former prisoners and current ones on special passes crowded around the television screen in the lobby of the Havana Riviera Hotel, where the exile group was housed.

Many of them, like Sergio Capdevela, 32, had waited there for days for news of their fates. Imprisoned from 1963 to 1969 for "subversive propaganda and sabotage" as a student, Capdevela is on "conditional liberty," meaning he can be put back in prison at any time to complete his 12-year sentence.

Despite the announcement that all former political prisoners would be permitted to leave Cuba as soon as another country offers to accept them, Capdevela was nervousas he pleaded his case to anyone who would listen.

"I haven't been able to sleep for a week," he said, waiting for the exiles to come. CAPTION: Picture, President Fidel Castro elicits bravos from an unlikely audience-Cuban exiles.