IN the early afternoon, the town of Jalapa, Nicaragua, bakes in the sun. Caliente. Hot. To the north, the border sizzles with warfare. Caliente. In Jalapa, they use the same word.

Counterrevolutionaries, the contras, have poured into Nicaragua from Honduras, intent on overthrowing the Sandinista government. Since the fighting began, hundreds have been kidnaped and others have been tortured and killed. The warfare has made virtual ghost towns of 90 of the 120 mountain villages. Jalapa, once a town of 9,000 people, has swollen to more than twice its normal size with refugees from the outlying communities of this northern region.

Jalapa, with its three airstrips, is a target for the contras.

So is Lisa Fitzgerald.

The townspeople she works with heard it first, on the contras' clandestine radio station. Fitzgerald, an American nun working in Jalapa, was threatened by name in a broadcast.

"I think it's a lot of hot air," says Fitzgerald. "It's meant to scare people, to scare people into leaving the area. I don't think it's added to my sense of caution."

Besides, she argues, it would be impolitic:

"I think now they're trying to reform their image. They're getting excellent publicity now," she says, with a half smile, of the American-backed contras. "The president called them 'freedom fighters.' It doesn't help their freedom-fighting image to be polishing off nuns--to put it crassly."

Nuns, as she well knows, are not immune to the warfare in which they have often found themselves. Four churchwomen were killed in El Salvador 2 1/2 years ago, and, despite U.S. pressure on the Salvadoran government, the five Salvadoran national guardsmen arrested in the case have yet to come to trial.

But Lisa Fitzgerald has never walked away from a fight. Not that she's doing the fighting, she adds quickly. It's more that she's never walked away from the fighters.

She is 39, a post-Vatican II nun who has spent more time in street clothes than in a habit. A graduate of Manhattanville College in Purchase, N.Y., she joined the Religious of the Sacred Heart--the order that formerly ran the school--after she graduated. She is a lawyer who worked for Greater Boston Legal Services and then as a Massachusetts assistant attorney general. In the Philippines, where she spent six months preparing for her final vows, she lived with a family whose food came from scavenging at a garbage dump behind a U.S. military base.

She arrived in Nicaragua 1 1/2 years ago to teach farmers and to train other teachers. Since the rebels' push to win Jalapa, she has become a keeper of facts about the casualties, a bandager of the wounded and a comforter of the townspeople.

"Our work is essentially work with the church," Fitzgerald says of the three nuns and three priests in Jalapa. "I don't think we became too involved in this. We're not Nicaraguans. We don't actively support the military--on either side. But for us to leave there now would be an act of cowardice."

She has left only briefly. She came to Washington this week to speak to members of Congress about what she has seen.

"Our experience in Nicaragua just didn't jibe with a number of newspaper accounts of what's going on on the border, and we felt it was time to make that known," she says.

"The people who've been injured or whose hopes have been crushed are poor, poor people, and in no way a threat to U.S. interests," she says, with a trace of a tremor in her voice. ". . . For us to be threatened by a nation of 2.5 million people and for us to arm a brutal group the contras that has a terrible reputation in Nicaragua is not only cruel, it's shortsighted."

Are the Nicaraguans she sees Communists?

"No!" she says incredulously. "They're ordinary people. For the most part, they're Catholics."

She is tan and muscular with deepset blue eyes. She wears a plain watch and a thin silver band on her ring finger. She focuses much of her attention on the burning cigarette in her hand as she describes the war that has raged around Jalapa--a town where the level of tension, according to her, "is already almost psychologically unbearable."

The stress of living in a town with a bunker mentality became apparent even in her own house. Minor annoyances like who left the dishes in the sink provoked quarrels.

Fitzgerald and the two nuns used to cook for the priests each evening. "We hate waiting on the priests for dinner," she says. "It's trouble to find food. There were some discussions about this--like, 'Shouldn't we share these responsibilities? Hey, boys, we're not your mothers.' " Fitzgerald solved this problem last March. "We divorced them," she says with a cool grin. "Now, they eat in their own houses."

She is the only American; the others are Mexican and Spanish. "I think one of the hardest things is for me to pray," says Fitzgerald. "Everybody speaks Spanish and when we pray, it's hard to express those feelings in another language."

Fitzgerald keeps a journal listing the dead and the missing. "We only do it so we can have a record," she says, pointing out that she only records what she can verify through eyewitness accounts or actual bodies. "There're at least four reporters coming into Jalapa every day," she says, "and they want information. We have it and we just type it up."

In the town of Jalapa, the people are preparing to defend themselves. They have dug a trench around the town, and men take turns on guard duty for three-hour shifts at night. Inside the town, women patrol with guns.

"They do that every night," says Fitzgerald, who neither patrols nor carries a weapon. "When things are more serious, we usually get word from the army. Then people are asked to keep the lights on in their houses--so the whole town is lit up--and their doors open at night to keep an eye on things." Every house has a whistle to be blown if something unusual is spotted. Fitzgerald and the two nuns she lives with offer bread and coffee to the people making rounds. One room in their house is used as a medical station since the town hospital is filled with the seriously injured.

"It's not scary," she says. "It's tiring."

She adds, "One night, I heard automatic rifle fire. I was the only one awake and I thought, 'Gee, I don't want to be listening to this alone.' " She chuckles. "So I woke up the others."

Her days now are filled less with teaching and more with visiting people, searching out information and passing on what she can confirm. She reads what newspapers she can find and, on her shortwave radio, she picks up National Public Radio better than the Nicaraguan stations. For diversion, she reads mystery stories and works in her garden, growing lettuce and struggling with zucchini, carrots and spinach.

She has brought a carefully detailed map of the Jalapa region, the municipio, drawn by her and another nun "who's a little more artistic." On the map are names of communities with little cone shapes beneath--green for the communities in the mountains, most of which no longer exist, yellow for communities along the main road that runs down the middle of the region. There is a big red circle for the main town of Jalapa.

"Most of the people from El Corozon have left," she says, touching the yellow mark next to the town's name. "Most of the people from El Carbon have left. In most of these communities," she says, pointing to the green marks, "one community leader has been killed. In one town, 30 were kidnaped . . ."

Fitzgerald and her religious colleagues used to make occasional trips around the area, putting the others in a state of nervous tension until they returned. Eventually, this recurring problem was solved:

"The army asked us not to go anymore," she says.

The most unnerving trip, however, is one she still makes, on the main road from Jalapa to Ocotal. It is en route to Managua, the capital.

The contras, she says, "are not discerning about who they pick off the road." She hitched a ride a couple of weeks ago with a farmer in a pickup truck to get to the airport in Managua. There were several other riders.

On trips like this, she says, "Nobody talks. You just have your eyes glued to the hills. It's kind of beautiful. Very green."

And dangerous. "I think the height of my fear," Fitzgerald says, "was on this road."

Her parents, who live in Upstate New York, find it difficult to comprehend why she stays. "Their impression is that Nicaragua is a Communist country, and they don't understand why the church would choose to remain there."

For Fitzgerald, though, it's rather clear. "The church would lose if we left them," she says. "Part of Vatican II is to be with people--not just in the best of times."