The first change in her life came with her brother's death.

It was 1967, and her 21-year-old brother had been killed while fighting as a Sandinista guerrilla in the mountains of Nicaragua. She decided she had to join their cause.

It would not be easy: Her parents didn't want her involved.And at first, the guerrillas didn't want her either. Few were trusted.

She went to a cousin involved in the Sandinista effort to overthrow the longstanding regime of Gen. Anastasio Somoza. She told him she wanted in.

"He said to me, "I don't know what you're talking about,'" she recalls. "I said, 'You know what I'm talking about, so don't pretend.'"

Thirteen years later, Rita Delia Casco-Montenegro raises her thinly arched eyebrows -- a preface to the story of how her life was twice transformed in 32 years: how the young, privately educated, carefully sheltered daughter of a Nicaraguan coffee-grower became a globe-trotting revolutionary, buying guns, smuggling money for her cause and living underground.

And then, with the war won, how the guerrilla agent, a former American University graduate student was transplanted to the glitter of embassy row as an ambassador to the Organization of American States.

A year ago, she and her Sandinista compatriots could barely get into the OAS discussions on the crisis in Nicaragua. A few months later, she was back as an official ambassador. Six months after that, she was chairing the Permanent Council of the OAS, a position that rotates alphabetically every three months. Last April, it was Nicaragua's turn.

Her post was once held by the former dean of the diplomatic corps, Guillermo Sevilla-Sacasa, brother-in-law to the ousted Somoza. In the huge high-ceilinged OAS office, which was hers for three months, the windows run the length of the office wall and sunshine streams in. She is tiny and seemed almost dwarfed by the large leather armchair.

When she traveled as a Sandinista, she says, "I lived on nothing," borrowing a dress when she had to make a fundraising speech. Now she dresses in trim jackets and high heels. After the revolution, "I wanted to work in the agrarian reform program," she says. "That's what I asked for. But they said, 'You have so much experience traveling abroad and talking with people, why don't you do this? So, I said 'Fine, as long as it's not forever.'"

For the three months that she chaired the Permanent Council she was driven around daily in the black limousine -- equipped with a phone and tagged OAS 1 -- provided for the chairman of the Permanent Council. "It used to bother me," she says, "but not now -- you save a lot of time. I'm very good friends with the driver. We talk all the time. He's Nicaraguan."

She stretches out and grasps the arms of the chair firmly, smiling sheepishly as she tells her story. Boarding School & the Barrios

She was 11 when she left home for Catholic girls' boarding school in Matagalpa. The students wore uniforms at Colegio San Jose, rose every morning for 6 a.m. mass, and were not allowed to leave the school grounds. "It was a little like a prison," she says, "except that I don't remember it like that."

Sunday mornings, the students went out to the barrios to teach religion. The people she taught made do with very little, and religion meant everything to them. What she saw, she says, was "social injustice." Unlike her brothers and sisters who shunned boarding school, she stayed for five years. "The one thing I learned was discipline," she says. "And I became more independent, it was easier to leave home, leave the country, leave my parents."

Once during her years in Matagalpa, university students marching in Leon were killed. A march was organized to mourn for them. She and her friends wanted to go, but were afraid they would be expelled. She went to a sympathetic monsignor and asked him to "negotiate" with the nuns. He did, and she and her friends marched. "We were not political then," she says. "We were just being human. We were Catholics." War Stories

But by 1967 she was 19 and things were different.

After telling her cousin that she wanted to join the cause, word traveled and she eventually got her first job -- carrying a message from one member of the organization to another.

At first, she says, she was just reacting emotionally to her brother's death. But her politics were changing too, she recalls, as she was beginning college at the University of Maryland in College Park. "There was the Vietnam War, I was studying sociology," she says. "I began to understand the roots of the struggle in Nicaragua." In 1971 she graduated with a bachelor's degree in social sciences and economics. In the early '70s she worked at American University on a master's degree in economics that she never completed.

At first she worked only sporadically for the Sadinista Liberation Front.

Still, between college and last July -- when the Somoza government was overthrown -- she says she delivered hundreds of messages. She also recalls raising several million dollars, speaking to groups all over the world, buying weapons in various places and hopping from one continent to the other with over half a million dollars in cash for the purchase of medical supplies, weapons and food.

During the late '70s, she spent about three years traveling, searching everywhere for weapons. "We bought in America, Europe, all over -- legally and on the black market," she says. Customs & Conversation

When Casco-Montenegro talks of those days, her face lights up, her laughs are quick and easy, her tales pungent and intense: a woman warrior savoring her revolutionary war stories.

"Maybe you started asking for a little part of a weapon," she says, holding out thumb and forefinger. "Or, maybe, you would go in to a place and ask to talk to the manager or the owner. I would say, 'Oh, I'm interested in an M-1. And I need to take it out of the country.' The owner would throw up his hands and say, 'Oh, I don't do that!' I would say, 'I know you don't do that, but you know what's going on in Nicaragua . . .'"

Her voice grows quiet and calm, her smile confident. "I would tell him, 'I don't want to buy five or seven. I want to buy . . . quite a few.'"

She never bought weapons in any place where she could be identified -- never, for example, in Maryland. She never told her parents what she was doing. "That would have been silly," she says. "I think they would have approved but they wouldn't have been vocal about it," she says.

In the last two years of the Sandinista effort, she was never in one place for more than 30 days. She traveled in and out of Latin America frequently, living at different times in Panama, Honduras, Costa Rica and Nicaragua. She left an American husband behind part of that time in Washington and Panama. Her daughter, now 5, was shuttled between her husband, her friends and herself. "She's gone through a lot," says Casco-Montenegro.

For most of that time, she was separated from her American husband, whom she met at college and who lives in the area now. The couple is in the process of getting a divorce.

She says she once transported $700,000 in her handbag from Spain, through the Venezuela airport, through customs -- without being stopped -- then around Venezuela for two days and out of the country again. "I would just talk to the customs people, smile and be nice," she says.

Once, she recalls, two male companions traveling with her were stopped going through a metal detector. They were found to be carrying $2,000. But "I knew nothing would happen to me," she says. "I just knew it. I was always very careful."

And she was apparently a good talker. In Europe and Latin America, she entreated political groups and organizations to donate money, telling many of them that whatever money they already invested in the Somoza government never reached the people.

She says she never feared being killed -- "just arrested."

Now, with a home in Bethesda and her in Montessori school, she misses the Sandinistas she worked with. And she sounds as if she misses the excitement as well. In her off time in comparatively tranquil Washington she jogs and collects Nicaraguan art.

The parents she left long ago are, she says, among the large growers of coffee in Nicaragua, living in the mountainous Jinotega region. Since the revolution, they no longer directly export their coffee, but sell it to the government, which exports it. She says they have not suffered monetarily from the revolution. "You have to understand," she says. "Many people are still making a lot of money."

She sees her parents infrequently. But they bought a pony for their granddaughter and they worry about their daughter. "My mother thinks it's dangerous," says Casco-Montenegro. "You know -- people take diplomats hostage. She'd probably like me to do something else." The Polite Diplomat

The embassy world is new to her. She socializes willingly, cheerfully, learning the diplomatic ropes that many others have spent whole careers climbing.

"We're at a very important point of consolidating our government," says Roberto Vargas, cultural attache to the Nicaraguan embassy. "This is the period when we make it or break it, so whoever represents our government is very well respected [in Nicaragua]. We want to do as smooth a transition as possible."

Not surprisingly, her style of diplomacy is nothing like that of her predecessor, Sevilla-Sacasa. Casco-Montenegro is almost shy -- "very polite, very humble, you never have the feeling she's trying to pull rank," says OAS secretary-general Alejandro Orfila. "I've never heard a bad word about her."

Nor does she harangue for her cause. "She never abused her position [as chairman of the Council] to give out propaganda," says Eduardo de Zulueta, the permanent observer from Spain to the OAS. He and Casco-Montenegro presented their credentials on the same day -- Sept. 12, 1979 -- and have been good friends ever since. "She says exactly what she has to say. No more. No less. But that doesn't mean she can't be forward when she has to."

When asked if the Sandinista movement is communist, she intones the standard Sandinista reply: "That's part of the struggle -- to show we are not communist, we are nationalists. We are pragmatists."

As chair of the Permanent Council she called meetings and presided at them, sometimes coaxing one party to compromise with the other on a resolution. tAs an ambassador, she attends lots of committee meetings, does her homework, listens to her diplomatic elders. And she lobbies her colleagues for a bigger piece of the $80-million OAS fund (the U.S. contributes a little over half) available to member countries.

In short, she has become the consummate diplomat.

"She's very forceful, but never antagonistic," says Orfila. "She's probably gotten much as she has for Nicaragua because of the way she is."

Of course, he says, "She's a Sandinista and she reminds people of it every now and then."