A walnut-sized baboon heart beat longer than any in animal-to-human-transplant history, but in Baby Fae's last five hours the alien organ was so poisoned by failed kidneys and so weakened by a previous battle against rejection that it could not keep even her tiny body alive.

Despite her death Thursday evening and widespread medical skepticism over the transplant, her surgeon said today that he would pursue his goal of saving doomed babies by giving them animal hearts.

"Baby Fae has opened new vistas for all, including the as yet unborn infants with similar lethal heart disease," said Dr. Leonard L. Bailey, 41, who headed the baby's surgical team at the Loma Linda University Medical Center here.

Bailey, who experimented with 150 animal-to-animal transplants before attempting the unprecedented implant in a newborn human, said he planned to perform more such transplants on children with hypoplastic left-heart syndrome after assessing the Baby Fae case.

He indicated that the 1-month-old infant's death had not quenched his hope that readily available hearts from baboons, who reproduce easily in captivity, will eventually save hundreds of babies for whom few human donors are available. "The Baby Faes and their parents are the real pioneers in this quest to enrich our quality of life," Bailey said. "Her unique place in our memories will derive from what she and her parents have done to give rise to a ray of hope for the babies to come."

In a voice that often broke with emotion, Bailey described the dark-haired baby's last crisis before her death at 9 p.m. PST Thursday.

He said the baby's 23-year-old mother and 35-year-old father, who reportedly were not legally married and had separated before her birth, were with the child at the end. They have requested anonymity.

"They spent a great deal of time alone with their baby" after her death, Bailey said. He said they were grateful for the chance their child had and encouraged him to do an autopsy to aid his research.

"Carry it on. Don't let this experience discourage you," he said the mother told him.

Bailey said that the autopsy showed the heart to be inflamed, an indication that an effort by the body's immune system to reject the baboon tissue weakened the organ during the past week. But he declined to blame a new drug used to suppress the immune reaction, cyclosporin A, for the kidney failure that precipitated last night's crisis.

He said it would take some time to assess the results of the experiment at this institution 60 miles east of Los Angeles. He said the kidneys, which had been malfunctioning since the weekend, could have been affected by the weakened heart function or by certain antibiotics used to try to protect the baby from random diseases.

Bailey and Sandra L. Nehlsen-Cannarella, an immunologist brought from New York to advise on the transplant, said they were encouraged that Baby Fae reacted to a transplant from another species no more strongly than would have been expected with a human-donated heart. Both have argued that a newborn's undeveloped immune system may increase the chances of success in such operations.

Bailey, who grew up in Takoma Park, Md., told reporters at a packed morning news conference of watching the children's television program, "Mr. Roger's Neighborhood," with one of his young sons this morning.

Football star Lynn Swann was telling the host, Fred Rogers, that he had lost a game. "But did you play your best?" Mr. Rogers asked. "Yes, we did," Swann said. "Then you won," said Mr. Rogers. "Then you won."

Only three attempts had been made to keep a human patient alive with an ape heart, once in Mississippi in 1964 and twice in South Africa in 1977. None of the three adult patients lived longer than 3 1/2 days; Baby Fae lived more than 20 days after her transplant Oct. 26.

As described by Bailey today, Baby Fae's last day began with doctors still optimistic they could reverse the lingering effects of a rejection episode that they thought peaked Monday evening. Signs of rejection agents in the child's blood had declined. Her heartbeat and blood pressure were normal. She was pink and active without a fever.

Her kidneys still were not producing enough urine, and that indicated that the heartbeat was not as strong as it should be. But doctors felt they could solve the problem by cutting off some antibiotics and readjusting her other medication.

Doctors here said Wednesday they had increased her dose of the immunosuppressant cyclosporin A to guard against future rejection episodes and did not think the drug would harm the kidneys, as several outside experts had predicted.

"Cyclosporin has been touted as a drug that can kill kidneys," Bailey said today. But, he said, "I'd ask you not to make too much of that connection" because his research indicated newborns do not react to the drug as adults do.

The regular 4-p.m. meeting of the baby's medical team Thursday "ended on an upbeat note," Bailey said. "We were still enormously concerned with the renal function . . . but we had not yet gotten to the point where we thought we ought to be aggressive."

Anyone who has treated small babies, however, knows that "they change very fast," he said. Within two hours, the kidneys were failing rapidly. At about 7 p.m., doctors attempted peritoneal dialysis, flushing the abdominal cavity with fluid to try to cleanse from the blood wastes that the kidneys no longer could handle.

The heart, reacting to the blood's changed chemical balance, began to beat irregularly. Drugs were used to try to stimulate the heart, but Bailey ruled out using a pacemaker to jolt the organ electrically.

Doctors here had speculated that in a crisis they might try to implant another baboon heart, or a human heart if one could be found, but Baby Fae's deteriorating condition made her "not an acceptable candidate" for a new heart, Bailey said. Replacing her kidneys would have been "a surgical tour de force" on a baby that weighed only 5 pounds, 3 ounces, he said.

A final attempt to revive the heart by massaging the baby's chest failed, and the heart stopped at 9 p.m.

Bailey said that a full report would be released eventually "to the scientific community." University Vice President Edward C. Wines cut off the news conference after an hour. Requests for followup interviews were declined.

In his only previous appearance before reporters, two days after the transplant, Bailey gave apparently erroneous information on the success of a procedure developed by Dr. William Norwood to try to fix hearts like Baby Fae's. In Boston and now Philadephia, Norwood has reported a success rate of up to 50 percent in a preliminary operation to reroute circulation so the child can live even with the left side of the heart virtually missing.

Bailey, who reportedly had tried the procedure once and failed, told university officials that Norwood's technique had not been duplicated by any other doctor. But Boston doctors have reported several successful operations this year that they say may allow afflicted children to lead a normal life, with limits on physical exertion.

A reporter who attempted to ask Bailey what he had told Baby Fae's parents about the Norwood procedure was barred by a guard from following him out of the news conference.

Friends and relatives of the child's mother have indicated she was told all the options.

Bailey indicated today that the parents had an attorney collecting media offers for their exclusive story. He also indicated they would attend a memorial service Saturday at the university.

Outside the conference auditorium, three protesters of animals' use in medical experiments held up a sign saying, "Bailey's Gift to Baby Fae: 3 Weeks of Suffering."

Bailey said today he felt that "Baby Fae suffered a good deal more before I saw her than after."