In a five-hour operation, physicians here have replaced the fatally defective heart of a 2-week-old girl with the heart of a young baboon in the first transplant of its kind involving a human infant.

The doctors said they plan four more such experimental transplants.

A spokesman for the Loma Linda University Medical Center said today that the infant, called "Baby Fae," was awake, "doing well and making improvement" since the operation Friday morning. Dr. Leonard L. Bailey, who headed the surgical team, said the baby had no chance of survival without the operation.

The baby, whose identity has been kept confidential at the parents' request, was born Oct. 14 with hypoplastic left heart complex, a severely underdeveloped left side of the heart. Bailey said about one infant in 12,000 has the condition. None is known to have survived more than a few weeks.

Four earlier attempts at heart transplants from non-human primates to humans -- in Mississippi, South Africa and Britain -- ended in the patients' deaths.

The Barstow Desert Dispatch quoted Baby Fae's mother as saying "she wouldn't be alive if she weren't in the hospital."

Loma Linda hospital spokesman Dick Schaefer said members of the transplant team "got tears in their eyes and some of them embraced each other" when the heart of the young baboon began to beat in the child's chest about 11:35 a.m. Friday.

Schaefer said today the baby, who weighed 5.07 pounds at birth, is "remarkably stable," with "good blood pressure, opening and closing eyes. Her mother is giving her love." She is listed in critical condition, which spokesmen said is usual for any patient recovering from heart surgery.

Sandra Nehlsen-Cannarella, a New York immunologist called in to assist in the operation, spoke of "an overwhelming feeling of accomplishment and satisfaction." She said "it was a big step just to get her out of that operating room and into her little bed looking pink and healthy."

Friday's cross-species operation, called xenotransplantation, is the first of five the hospital plans to perform. It spotlights advances in drugs to suppress organ rejection and in tissue typing to determine the best animal organ donors.

Doctors here said infants have a limited capacity for rejection of foreign tissues, thus increasing the potential for success of such an operation.

But the baboon transplant is expected to add fuel to a national debate on use of animals in medical experiments.

Some experts cautioned today that the baboon heart transplant is a highly experimental operation that has not produced a long-term survivor.

"The unknowns are so great, in terms of whether the body will be able to tolerate another species' heart, that one has to look at it as an experimental procedure in a desperate situation," said Dr. Nevin Katz, associate professor of cardiovascular surgery at Georgetown University Hospital in Washington, D.C.

"There's so little experience, especially in young children, that we just don't know how well they can tolerate it," he said.

Katz said the procedure is "appropriate . . . in a child who would die otherwise." But he said that even a baby's less developed immune system might reject the foreign organ, immediately or as the child grows.

Arthur Caplan, an expert on medical ethics and an associate at the Hastings Center in Hastings-on-Hudson, N.Y., said the operation is certain to cause controversy.

"I don't have a particular moral objection to the transfer of material from one species to another, but there are a lot of people who do," Caplan said. "There are certain religious groups that see species lines as inviolate."

Caplan said others would object because of "the symbolic significance of the human heart as the seat of the emotions, of the soul." He said these critics believe "that it's wrong to use animal parts as a way of keeping ourselves alive, that it's a violation of human dignity."

Georgetown's Katz said the use of baboon hearts "could potentially be a very important advance" because there are so few suitable donors for infants. "There are very few infants who die from noncardiovascular causes and are in a position to donate the heart," he said.

Katz said a baboon's heart is "somewhat human-like" and "a reasonably good match" but is made of different tissue.

Dr. Bailey, who was born in Takoma Park, Md., and attended Columbia Union College there, said, "We're not in the business of uselessly sacrificing animals. But we're forced to make a choice. We can either decide to continue to let these otherwise healthy human babies die because they are born with only half of their heart, or we can intervene, and, in so doing, sacrifice some lesser form than our own human species."

In a series of questions and answers distributed by officials of this 546-bed, Seventh-Day Adventist facility 60 miles east of Los Angeles, Bailey said that if the medical team is successful in suppressing rejection of the baboon heart through a new drug called Cyclosporin-A and other immunosuppressants, such as steroids, the child has a chance to lead a normal life.

The baboon and human hearts are nearly identical, a university spokesman said. Bailey said he thinks that if the baboon heart isn't rejected it will grow as the child does.

"There's nothing to suggest that the heart won't respond to the internal milieu of the baby," Bailey said. "That's . . . a well-recognized phenomenon in kidney and liver allotransplantation transplants within species in animals."

Bailey, who received nearly all his medical training at Loma Linda University, said he has performed more than 150 heart transplants between newborn animals in the last seven years. He said "there is evidence that the chimpanzee, orangutan or gorilla may be a better donor," but "they are either an endangered species or don't procreate well in captivity."

Spokesmen said the university's 20,000-square-foot animal-care facility had six young baboons that could have been heart donors.

One was selected after a series of tests, including a five-day lymphocyte test. "Many people smile when they hear we had a relatively compatible baboon for the baby," Bailey said. "But . . . by screening a group of donor baboons, it is possible to find immunological match that aids in selection of a donor."

Hospital officials said they had received dozens of calls praising the operation and two or three critical of the use of animals. "You know the baby can't live," one caller said. "Why are you putting this animal through this?"

The first heart transplant to a human from a non-human primate was performed at the University of Mississippi in 1964 when Dr. James Hardy placed the heart of an adult chimpanzee in the chest of a 68-year-old man. The man died within hours.

Three similar transplants in South Africa and Britain also were unsuccessful. In 1977, South African transplant pioneer Dr. Christiaan Barnard placed a baboon heart in the chest of a 26-year-old Italian woman, who lived four hours. Later that year Barnard transplanted a chimpanzee heart next to the failing heart of a 59-year-old Cape Town accountant, who lived 3 1/2 days with both hearts.

In 1978, Barnard told an interviewer that he had abandoned transplanting hearts from non-human primates to humans. "I stopped that," he said, "not because I'm so convinced I'm on the wrong track, but I got emotionally involved with the chimp."

The Loma Linda operation began at about 7:30 a.m. Friday when Baby Fae was taken from intensive care to the surgical suite of the medical center. To slow bodily functions and to make the delicate surgery easier and less traumatic, her body temperature was lowered from 98.6 to 68 degrees Fahrenheit with the use of a heart-lung machine.

At about 9:15 a.m., Bailey left the operating room to obtain the baboon heart, a procedure that took about 15 minutes, according to hospital spokesmen.

The child's chest was opened and her heart removed. The implantation of the animal heart took about one hour, after which the child's blood temperature was returned to normal, her new heart started and her body weaned slowly from the heart-lung machine, according to the hospital.

Hospital spokesmen said the operation profited from experience in the transplant of ape organs, including kidney xenotransplants performed in the early 1960s and the use of baboons in humans with liver failure.