Baby Fae, the dark-haired infant living on the beat of a baboon's heart, began drinking formula from a bottle today as she surpassed the survival record for animal-to-human heart transplants.

Listed in serious but stable condition, shown on a hospital videotape yawning and stretching, the world-famous 16-day-old girl was improving steadily and showed no signs of rejecting the walnut-sized heart four days after Friday's transplant.

"I'm more than surprised, I'm elated," said Dr. Theodore Mackett, a surgeon at the Loma Linda University Medical Center where the operation took place. "I'm surprised in capital letters because it's never been done."

Doctors and officials at this Seventh-day Adventist institution 60 miles east of Los Angeles continued to avoid questions about the decision to proceed with the experimental operation when more proven surgical repair procedures were available to treat the child's deformed heart.

A hospital spokesman said today that Dr. Leonard L. Bailey, the surgeon who advised the child's parents on alternatives to the operation and who performed the transplant, was responsible for erroneously reporting that a surgical procedure used in Philadelphia had not been duplicated at other hospitals.

Hospital officials said Bailey declined to meet further with reporters after a Sunday news conference at which he gave an incomplete account of the success of the surgical procedure, which is now said to have helped at least half the babies with hypoplastic left heart syndrome brought to hospitals in Philadelphia and Boston.

Bailey "is totally absorbed with the treatment of this child," Mackett said at a morning news conference. "He is not a publicity-seeker, and he is very sensitive on that point."

Animal-rights activists and some medical ethicists continued to question the operation, the first of what hospital officials hope will be five baboon-to-infant heart transplants.

Ronald Bayer, a bioethicist at the Hastings-on-Hudson Institute, said doctors here were wrong to pass up a proven surgical procedure. But Dr. Willem J. Kolff, inventor of the artificial kidney and a force behind the creation of the artificial heart, applauded the California experiment.

The previous longest-surviving recipient of a nonhuman primate heart, 59-year-old South African accountant Benjamin Fortes, died in 1977, 3 1/2 days after a chimpanzee heart was implanted alongside his failing heart. His surgeon, Dr. Christiaan Barnard, had placed a baboon heart in the chest of a 26-year-old Italian woman earlier that year, but she lived only four hours.

The first and only other known ape-to-human heart transplant occurred in Mississippi in 1964, when Dr. James Hardy replaced a 68-year-old man's heart with that of a chimpanzee. The patient died within a few hours. In 1975, London surgeon Magdi Yacoub kept a 14-month-old boy alive for 16 hours by connecting his circulatory system to the heart of a living baboon, but the operation was not a transplant.

Baby Fae, the name given the infant here to protect her and her parents' identities, is the second-youngest person ever to receive a transplanted heart. A 9-day-old British girl, Hollie Ruffey, lived for three weeks after she received a transplanted human heart in a London hospital in August.

With little financial or moral support from the rest of the American medical community, Bailey, 41, has worked for at least seven years to develop a way to use hearts from readily available baboons to save babies with fatal congenital defects. He performed more than 150 animal heart transplants before attempting the Baby Fae operation.

An infant heart available the day of Baby Fae's transplant was too big, doctors here said. Dr. John W. Mace, the child's pediatrician, was asked about a report that a better-sized heart became available in Kansas City two days after the transplant. He said there was no point putting the infant through another operation if her baboon heart was working well.

Doctors today rejoiced at their success in preventing an immediate rejection of the heart, which they attributed to the premature, 5.07-pound infant's underdeveloped immune system and the use of a new, sophisticated immunosuppressant, cyclosporin-A, along with steroids. They said that they anticipate an eventual setback as the child's body reacts to the alien heart, but that they do not know when it will occur.

Loma Linda University surgeon David Hinshaw said that if the child's heart begins to fail, another transplant of a baboon or human heart "is a distinct possibility."

Mace said the child's parents are happy with the experiment's success so far. He said that when the mother was asked what she thought of criticism of the operation in the press, she answered, "They don't know what they're talking about."