Dr. Christiaan Barnard's daring transplant of a baboon heart into a chronically ill 26-year-old woman yesterday failed when her own heart deteriorated and the baboon heart - about half the size of a human heart - could not carry the full circulation alone, the weary surgeon said today.

Barnard told a press conference that he had made a mistake in allowing the patient, who has suffered from heart disease since birth, to remain on the heart-lung machine for almost 10 hours.

"I've never seen a patient on machine for 10 hours - the limit is destroying the blood elements," he told reporters during a meeting at Groote Schuur Hospital in Cape Town.

The famous surgeon speculated that the operation "might have had a chance" if the patient's system had not been seriously weakened by keeping her on the heart-lung machine for so long.

Although a full laboratory report has not been completed, Barnard said there were no immediate signs of rejection of the baboon's heart as the cause of death, which occurred 2 1/2 hours after the 12 hour operation ended.

Bernard, the pioneer of the heart transplant operation, said he is still committed to the use of animal donors, although he added: "I think this is the last baboon transplant I am going to use. I think I will use chimpanzees next time." Chimpanzee hearts are larger than baboon hearts.

The South African heart specialist decided to attempt the baboon transplant in a desperate bid late in the operation after conventional reconstruction surgery on a faulty aortic valve failed and the patient, whose name has not been disclosed, face certain death because her own heart could not function without assistance.

Barnard showed surprised at the immediate failure of the radical transplant. He said today that the woman regained consciousness after the operation and both hearts appeared to be working well.

Groote Schuur Hospital reported this morning that the woman's husband had been admitted for treatment for shock.

In defense of the radical and controversial surgery, he added: "Because it was a new process and a desperate process, we decided to try everything we knew first. They can't fault me on that, that I didn't use something more conventional."

A human heart donor would probably have kept her alive, Barnard said. "But it is matter of availability. You don't have a donor just when you ask for it."

Barnard has had two baboons in isolation for several months in preparation for the operation. Both had undergone tests for blood type and to insure they were free of a common baboon viral infection fatal to humans.

The baboon was selected for two reasons: Similarity with the human anatomy and availability in South Africa. Gorillas and chimpanzees were also considered, but in February Barnard said, "I am afraid they are as scarce as human beings today, so we cannot use them. But in this country, the baboon is readily available."

Today, however, he revealed that Groote Schuur has purchased several chimpanzees from an unnamed foreign country - at a cost of about $17,500 each - for use in future transplants.

The animal transplant is the next step in his unique strategy, developed almost three years ago, to introduce a second, supportive heart to help strengthen the original heart so it would not have to be removed.

Barnard said the second heart is placed in front of the right lung, a locaution picked because the lung can expand around the new object without interfering with breathing.

Barnard has abandoned the single transplant - as have most doctors around the world - because of complications, rejection problems and new alternative treatment. South Africans are the only surgeons who have attempted the double transplant, which Barnard admitted is still viewed skeptically by surgeons elsewhere.

The animal transplant is considered a temporary measure with two followup options: remove the second heart once the diseased heart is strengthened and healthy, or install a humanheart as soon as one becomes available, preferably within three weeks.