A young couple living deep in the Sumatran rain forest near here is teaching pet orangutans how to return to the wilds. The reddish apes, whose name means "man of the forest," are considered prestigious pets by many Indonesians, although keeping them is illegal.

Cuddly and babylike when they're young, adult orangutans can weigh over 200 pounds and have the strength of three men. That's when their owners tend to get nervous and call for Rosalind and Conrad Aveling.

The Avelings, both 26, are British zologists who run the Orangutan Rehabilitation Project, organized by the Swiss-based World Wildlife Fund. The project involves curing the orangutans of any human-borne diseases they've picked up in captivity and then preparing them to live without human support in the rain forest.

But the rehabilitation of the apes is largely symbolic.

"Rehabilitating a relative handful of orangs isn't going to make much of a difference in the survival of the species," Conrad said. There are now guessed to be anywhere between 4,000 and 12,000 orangutans in north Sumatra, Borneo and the east Malaysian state of Sabah, their only natural habitats.

"The orangs are the figureheads for what we're really trying to do here," he continued. "Our main task is to try to preserve their habitat, to halt the wholesale destruction of the rain forest itself."

According to Conrad, who holds a doctorate in entomology, the study of insects, the Sumatran rain forest is rapidly being eaten away by oil exploration, voracious timber-cutting and inefficient farming.

"In another 10 years, 80 per cent of the forest will be gone," he predicted gloomily. "The consequences for the ecology of Sumatra and for worldwide knowledge of tropical rain forests could be disastrous."

With this in mind, the Avelings work from 7 a.m. to midnight on a variety of research projects concerning the dense, steamy jungles that comprise the 80,000-acre Langkat Reserve in northern Sumatra.

"Research is critical right now," explained Rosalind, who's a specialist in primates. "So little is really known about tropical forests, and it's essential that we move quickly . . . before it's all gone."

But the eye-catching part of their job is working with the orangutans. "It's what draws the visitors. Believe it or not," Conrad said the other day, "we get as many as 150 tourists out here on a Sunday."

It is a little hard to believe. Anyone traveling to the project site, five hours by rattletrap bus from the port city of Medan or three hours in a jeep over rocky roads, has to walk the final mile through a natural-rock tunnel, along a steep, hilly trail, parts of which were recently washed out in a monsoon deluge, and then cross a narrow but swift river.

The crossing is accomplished by first shouting for one of the three Indonesian forest rangers who work with the Avelings. One of them meanders down the hill from the rough hut where they live and bails out a pencil-thin dugout canoe.

Squatting in the dugout and hauling on a rope linked to a steel cable stretched across the river, the ranger floats over and takes visitors back. More often than not, the canoe is full of water once again by the time it completes the trip.

Then a scramble up the slope to where the Avelings live in a tiny concrete-block, thatch-roofed cottage. Conrad, blond and bearded, looks the part of a jungle researcher. But Rosalind, whose long brown hair and porcelain complexion give her a certain Alice-In-Wonderland look, seems oddly out of place.

Until the visitor realizes the infant she's cradling is an orangutan. "We got him when he was 1 month old." She said, "Loggers had shot his mother and were keeping the baby. The Indonesian conservation authorities confiscated him and turned him over to us.

"We named him Riang, which means 'cheerful,' because he was so happy even though he was shriveled and dehydrated and one of his fingers had been broken." Rosalind paused for a moment and then added with a smile. "But we weren't terribly cheerful when we had to get up in the middle of every night to give him his formula." Riang is now 4 months old, sleeps through the night and no longer wears diapers. But he still drinks canned baby-formula milk.

Although she was obviously attached to Riand. Rosalind said she'd have no trouble sending him off to the jungle when he got to be 9 months old or so. "After all," she said, "that's what this project is all about."

Rosalind has shown herself to be tough when the welfare of the orangutans is at stake. Recently, she said, while on a trip to a remote area in north Sumatra, she confiscated a pet orangutan from some loggers and brought it back, on a 16-hour bus trip.

"Actually, we have no authorization to confiscate the animals," she admitted shyly. "But I bluffed my way through. It wasn't too difficult, because most Indonesians know it's illegal to keep an orang."

The Avelings are now training 12 orangutans to climb trees and to forage for their own bananas, leaves, bark and insects. Another five of the apes were being confined in quarantine cages scattered around the cottage.

"The orangs are highly susceptible to human diseases," Conrad said. "They can pick up anything from colds to tuberculosis, and it's terribly important that we don't introduce these diseases into the habitat." While in quarantine, the orangutans are innoculated for a number of diseases and dosed with worm medicine.

The length of the teaching process varies, depending on the age of the orangutan and how long it's been in captivity. Each morning, either Rosalind or Conrad and one of the rangers trudge up 400 steps hacked into the side of a steep jungle hill.

At the top of the hill the orangutans are waiting, swinging from trees and vines, for their share of the 150 bunches of stubby cooking bananas distributed each week. The apes drop onto the top of a large, empty cage where the bananas are handed out and then scamper back up into the trees.

Some of the animals, particularly those who've spent more than two or three years as pets, are reluctant to climb. In these cases, Conrad and the rangers have to pretend to threaten them - swinging sticks, stamping on the roof of the cage and shouting in order to scare them up into the trees. If they stay on the ground, the easily fall victim to predators.

"We play at psychology," Rosalind said. "I'm the gentle one and Conrad is the bad guy. That way, the orangs don't become overly insecure."

Orangutans have a number of near-human physical and psychological traits, the Avelings noted. Their gestation period is nine months. They live to 35 or 40 years and attain sexual maturity between 8 and 12 can acquire the intelligence of a 5-year-old child.

Conrad told a story about forgetting his 35 mm camera at the feeding station one evening. "When I went back next morning, the thing had been taken completely apart," he said. "Not smashed, but completely unscrewed.

Orangutans live in trees and make fresh nests each night and often just to take a midday nap. Mothers nurse babies for 18 months and keep children with them until they're 3 to 4 years old. "It's not at all unusual for a frightened 2-year-old to return to its mother for nursing when it needs comforting," Rosalind said.

But unlike gorillas and chimpanzees (other members of the ape family that bear some human characteristics) the orangutans are loners in the forest. "This makes rehabilitation easier." Conrad said, "because there's no problem with fitting them into a social structure."

"After an animal has stopped returning regularly for feedings for three months or so, unless he or she just disappears, we try to take them into the deep forest and release them," he said. Last month, they transported several animals this way on a helicopter contributed for the day by the Mobil Oil Co.

"Considering that the oil companies are prime offenders in the destruction of the habitat, I guess it's good public relations for them," Conrad said.

Although the Avelings mourn the erosion of the rain forest, they're pragmatic. "I know human advance must come first," Conrad said. He noted, for example, that the Indonesian government' policy of transmigration, moving thousands of families from the overcrowded island of Java to Sumatra, would make a major contribution to destroying the jungle. "So will the Sumatran highway, when it's completed," he said. "But that's called progress."

The highway, which is intended to open up the interior of Sumatra, Indonesia's largest island, will cut through part of the rain-forest reserve. Recently, Rosalind said, a U.S. adviser on the project contacted them about thie suggestions to reduce damage from the road to the jungle.

The Avelings have been here since last March and plan on staying another two or three years. Their predecessors, a Swiss couple, stayed only nine months, "evidently because they didn't like all the visitors," Conrad said.

The orangutan project was founded in 1973 by two Swiss women, Regina Frey and Monica Borner, and is funded mainly by West Germany's Frankfurt Zoological Society.