Doctors said today that they are considering a second transplant operation -- possibly of another baboon heart -- for Baby Fae if she rejects the baboon heart that has kept her alive the past six days.

Baby Fae, the longest-living human recipient of a cross-species transplant, is still "doing well, taking a little more formula and sucking vigorously on the bottle," said Dr. David B. Hinshaw, a professor of surgery at Loma Linda University Medical Center.

Hinshaw cautioned, however, that the seventh to 10th days after a transplant operation are the peak period for rejection of a new organ.

Baby Fae is being given cyclosporin A, a drug that suppresses the immunological system, thus reducing the likelihood of rejection. If she shows signs of rejection, doctors say they will first adjust her medication and then consider a second transplant.

"We are actively seeking a human heart," Hinshaw said. "Replacement with a baboon heart would be considered second."

A second baboon heart, like the one now beating in the infant's chest, would come from an original colony of six baboons in a Loma Linda laboratory. Hinshaw termed the second baboon heart an "obviously poorer match" than the first heart but a close enough match to be "reasonable."

Hinshaw said that, while cyclosporin A's suppression of the immune system does make a patient more vulnerable to infection, it "does not suppress the entire sweep of the immune system" and has been used successfully on kidney transplant patients.

Doctors at Loma Linda revealed today that Baby Fae has been receiving a pain medication, the dosage of which has just been reduced.

Dr. John Mace, chairman of the Loma Linda Medical Center pediatrics department, said the tabloid National Enquirer has offered to pay $10,000 to anyone who will reveal the baby's identity, which is being guarded at the request of her parents.