Dr. Christian Barnard, innovator of the first human heart transplant 10 years ago, is now preparing to use baboon hearts in human patients - a step likely to spark even further criticism of the already controversial treatment.

A bank of specially-prepared baboon hearts is already on stand by at Groote Schuur Hosptical here to be used on the first occasion a human heart donor is unavailable. The ape heart, however, would be used only as temporary device to sustain life until a human donor is available.

In an interview at the hospital, Barnard said the radical step is the next phase in his double-heart-or heterotopic - treatment, the process of transplanting a second heart to support the original diseased heart.

Barnard introduced what he calls the "piggyback heart" operation in November 1974. So far the surgery has been performed only eight times and only in South African hospitals.

"I would obviously prefer to use a human heart for this piggyback operation," Barnard explained.

"But where patients could die within hours after after a massive coronary, for example, or where you cannot wear them off the heart-lung machine, I would put a baboon heart in as a temporary device or until a human heart for transplant becomes available."

The move comes at a time when many doctors have abandoned the single heart tranplant because of complications, rejection problems and new alternative treatments.

And the use of animal hearts is likely to spark new controversy, Barbard admitted, because of problems with different blood and tissue typing. Baboons have no O blood groups and their tissues are incompatible with humans.

The animals on standby have been tested to insure they are free of a common baboon viral infection fatal to humans. But Barnard admitted that many of the tests he would like to make on the apes "are so difficult that they can be carried out only in the United States."

He added: 'We can only hope that we will be able to control the rejection for a few weeks until patient's own heart has recovered, then of course we will stop the rejection treatment and take the baboon heart out.

"I think the rejection problem will not be as great as if we used, for example, a goat or a cow. The rejection problem would be in the intensity range that we may be able to control.

"I don't think we will use the baboon heart for more than a few weeks. I hope in that few weeks, the baboon heart will help the patient's own heart to rest, that the patient's own heart will recover to such an extent that it can do without the baboon heart. Or if we find we can not manage without assistance, then we will use a human donor to replace the baboon heart."

Barnard said other members of the ape family have also been considered, such as the gorrilla and the chimpanzee. "But I am afraid they are as scard as human beings today, so we cannot use them. But as you know, in this country, the baboon is readily available."