Her own laughter catches Celia Guevara when she least expects it. It animates her face and quickens the tempo of her sentences. When she hears the sound of her own laughter, she stops.

The vase of wildflowers on the kitchen table of her borrowed Capitol Hill townhouse reminded her that it is May, and spring. And of her homeland Argentian, where it is May, and autumn.

"The change in hemispheres is confusing," she explained. "I was in Cuba, and I told a friend that psychiatrists say that it mixes you up. Something actually happens inside your head.

"And my friend said, 'It's because down there you're upside down too much. All the blood rushes to your head down there,'" The old pan-hemispheric joke, recalled unexpectedly, dissolved in girlish laughter. The laughter as quickly dissolved to a shrug.

"That's what's most difficult for me," Celia Guevara said. "To live, to go to a party, to visit a museum, to have a life. And then to remember Che. And to think of Juan Martin in his cell, and to know what the tortures are."

Ernesto (Che) Guevara, Fidel Castro's right-hand man, was celia's oldest brother; he found death and martyrdom in a Bolivian jungle in 1967. His portrait still adorns the streets of Paris, over the legend "Che Lives!" Juan Martin is her youngest brother, alive but imprisoned in Argentina.

"Juan Martin was a journalism student in Buenos Aires, but he became a truck driver. He delivered cheeses. He joined both the truck union and the union for food workers, and he was very active."

He was arrested one summer day in February 1975, in a house at which he was visiting. "The police put a hood over his head," Celia Guevara said, "and then they shot all around him with bullets. But they did not hit him. Then they took the hood off and showed him a small room, in which there were guns and bombs and documents."

Juan Martin Guevara was charged with illegal associations and "bearing Arms," by Isabel Peron's police. When the junta took over the next year, he remained in jail -- he was tried and sentenced to 12 years imprisonment.

"We have a lot of revolutions in Argentian," Celia Guevara said. "None of them like labor unions. Legally, he had no chance, because anyone can be accused of bearing arms. The police and the security forces are masters of the situation.

"I am not a famous person," Guevara said, letting her smile return. "But what I am doing is all I can. To talk to journalists everywhere. Because I cannont live my life, and be happy, when people disappear. When they are put into tiny cells and never heard from again."

Guevara said that more than 30,000 persons have "disappeared" in Argentina in recent years, and that at least 15,000 have been imprisoned.

Guevara is not a totally unbiased observer of Argentinian politics; such groups as Amnesty International have put the number of missing at up to 20,000. "Every Thursday the members of the Argentine Family Committee gather to make peititions to the junta for information about their sons and husbands who are gone. They are very brave women. Many of them have been kidnaped, because the junta does not want publicity about the disappearances."

Gen. Jorge Videla, the president of Argentina, has put the number of political prisoners in his country at less than 4,000. "I believe their are far more than even we say," Guevara said.

Celia Guevara is an architect by profession, and she has lived in Madrid for the last several years. "It is a normal job," she said. "I work on building houses in the Canary Islands. I am employed by the state."

But because of her family, a normal life has eluded her, she said.

"I cannot go back home. On the day I left Argentina, the police came and ransacked my house. They were there waiting or eight days for me to come back. But I knew I would not."

As early as 1966, her mother, who was then terminally ill, was arrested and imprisoned for a short time. "She was active in a liberal political party," Guevara explained simply. Her other brother, Roberto, found he could not practice law in Buenos Aires because of the family name, and he now lives in Spain, too. Her sister married a man who "became political" and was deported."She followed him to Cuba, where they live now."

Asked to put a word to the family politics, she chose "socialist." "Liberal," she added. She seems aware of the connotations of party titles, and their difference from country to country. "Communism," she feels, has a confused meaning: a party title in Italy or France, with a substantial following. "But in the United States, yes, it apparently means Russia," she said.

It was her brother Ernesto, by becoming "Che," who changed the family's course permanently. Celia remembers him mostly as the rest of the world does -- as the Castro "soldier" tracked and killed in the jungle.

"He was a friend of Raul Castro's,"she said. "Then Fidel came to Mexico, and they all went to Cuba. Che could have been an administrator, but he chose to be a soldier. Yes, he was killed -- by an officer, that's all I know. There were many films of the body. I am sure it was Che." It is said that part of Ernesto Guevara's hand was preserved to prove his identity by fingerprints. "I have heard that," she said. "I don't know if it's true."

Celia Guevara's attempt to cast light on Juan Martin's imprisonment has lately taken her through Europe, and to New York, to which she returns this week after her stay on Capitol Hill. Then to Canada, "where we have many friends, many enemies of the junta."

Che Guevara saw the United States as the chief source of imperialism for the other Americas, a position Fidel Castro still maintains. But Celia Guevara apparently sees no irony in her visit to Washington. Because her great-grandfather was a native Californian, she has visited the office of Rep. Ronald V. Dellums (D. Calif.). Otherwise she speaks to reporters, groups interested in South American affairs and anyone else who will listen.

"I do not know that my travels are useful," she said. "But there is some hope that Juan Martin's sentence could be reduced if many people hear about his confinement. I know of one prisoner who, because his case was known in Madrid, was released early because the king of Spain came to visit Argentina."

When Celia Guevara thought she had finished telling of her brother's plight, her laughter returned. She talked about the few movies she has been able to see -- "The Tin Drum" and a film of Werner Herzog's starring Klaus Klinski. "It was that man who looks like a cadaver," she said, amused."The blond hair, the face sunken. Very ugly. But a wonderful film."

"I must make some coffee," she said, springing toward the stove. "In Argentina and in Cuba, coffee is very important, but mine never turns out well at all."

A moment later she laughed again, looking at her brimming cup. "You see? There are grounds floating, I have never had the time to learn of these simple things."