Seven years in rat-infested Angolan prisons for being a mercenary have not changed Gustavo Grillo's romantic -- perhaps scheming -- heart.

Just hours after he and two other Americans were freed in Lusaka, Zambia, in a complex swap involving six nations and about 100 prisoners, Grillo, 33, said today that he could under certain circumstances become a mercenary again. He added that he was also interested in going "back to Angola as a businessman."

"I'm a very romantic adventurer," he said early this morning on a flight to Paris en route home to New Jersey . "I like to warm my hands in the fire of life," he added when asked about becoming a "dog of war" again. Also on the flight were fellow mercenary Gary Acker, 28, of Sacramento, Calif., and pilot Geoffrey Tyler, 33, of Seabrook, Md.

According to Grillo, Tyler is also an "unserious guy," which helped him to survive his 21 months in prison after crash-landing a plane he was flying over southern Angola.

Acker, who was captured with Grillo in February 1976 while fighting with a CIA-backed faction defeated in Angola's civil war, "is very serious, too serious. He's very bitter," Grillo said.

Conversations on the plane with the two other American prisoners released in the deal bore out Grillo's appraisal.

Although all three men appear to have come out of the ordeal in reasonably good health, Acker seems to have been scarred the most. With his all-American looks, it is hard to think of the 28-year-old Vietnam veteran from Sacramento, Calif., as a mercenary. Acker declined to talk about his imprisonment and would only reiterate the others' hope that seven British mercenaries still held under long prison terms would also be released soon.

Unlike Grillo and Tyler, he refused to take any clothes from his captors when he was released early Tuesday morning into the custody of the International Committee for the Red Cross.

He was dressed in a combination of clothes from his British cellmate John Lawlor and his former cellmate, South African soldier, Johan Van der Mescht, who was traded for a Soviet spy in May.

Tyler said the Angolan prison guards forced them to strip and stack all their possessions in small piles before their release.

"They took small things, including Gary's personal letters. He raised hell and they tossed them into the plane at the last minute" before they departed, Tyler said.

Acker was only 21 when he answered an ad in Soldier of Fortune magazine to become a mercenary for the National Front for the Liberation of Angola. It was disclosed in congressional hearings that the Front received the bulk of $31 million spent by the CIA in 1975 and 1976 in unsuccessful attempts to prevent the Marxist Popular Movement for the Liberation of Angola from gaining power.

Grillo was sentenced to 30 years in prison and Acker to 16 years in a trial held by Angola in June 1976. A third American, Daniel Gearhart, of Kensington, Md. was executed along with three British mercenaries.

Grillo refused to talk about his severe criticism of the United States during the trial where he called U.S. society "a monster," a society of power seekers, status seekers, waste makers where the weak get weaker and the strong get stronger.

At the time, reporters at the trial interpreted the remarks as an attempt to avoid execution since there were allegations that he had killed some Angolan soldiers. Gearhart apparently never fired a shot during the four days the three men were in Angola before being captured.

Although Grillo talked some about his future plans, he was reticent to speak much about his imprisonment.

"There's no way I can put seven years in a few words," he said.

"I was never mistreated. They gave me the best they had. They didn't give it to me because they didn't have it." He had an operation on his left leg for a bullet wound suffered when he was captured but will need another one in the United States to try to correct his slight limp. He walks with a cane.

He also declined to compare his imprisonment in Angola with 18 months he served in the United States for armed robbery.

"I don't want to get into something deep. I don't like to talk about those things," he said.

He also acknowledged another reason for his reticence to give details of his experience.

"I don't want to be rude, . . . but I want to save myself. Maybe I can sell my story."

"I have plans for the future to make money in Angola," he added as the airliner neared Paris. "Why not. I'm going to throw seven years away?" He is aiming for the import-export trade, he said.

"At times, prison is the best place to be to learn things" about a country, he said. "Where would you send people to learn ? It's not going to be in the street."

A Marine platoon leader at the battle of Hue during the 1968 Tet Offensive in Vietnam, Grillo said it was impossible to compare the fighting in Vietnam and Angola.

"It would be like comparing Beluga caviar with chewing gum," said the 6-foot 4-inch Grillo, who was dressed in an ill-fitting Angolan safari suit unbuttoned to his chest.

The U.S. government found out about Tyler's imprisonment when Grillo and Acker told an Italian consular officer who visited them periodically that there was another American in the prison. Italy represents U.S. interests in Angola which does not have diplomatic relations with Washington.

In the delicate, almost two-year negotiation for the three Americans' release, there were four false starts which raised their hopes only to let them down.

The latest was about two weeks ago when the bodies of three South Africans were loaded on a Red Cross plane as part of the intended swap. For unknown reasons, only two bodies were sent in the finally successful exchange yesterday.

The worst letdown, according to Tyler, was last Dec. 21 when he and Grillo were taken to the airport in preparation for departure only to be returned, apparently because the Angolan government was not releasing Acker.

"December 1981 was an awful Christmas," Tyler said.