LOMA LINDA, Calif. (AP) _ A Canadian newborn whose fatally flawed heart was detected in the womb was given a new heart in an unprecedented transplant operation within hours of his birth by Caesarean section.

Paul Holc was in critical but stable condition early today at Loma Linda University Medical Center, hospital officials said. Previously, the world's youngest heart recipient was 4 days old.

Paul came into the world at 10:54 a.m. Friday, a 6-pound, 6 3/4 -ounce redhead diagnosed through an ultrasound examination in August as suffering from hypoplastic left heart syndrome, an underdevelopment of the heart's left side.

At 2 p.m., he was taken into 4 1/2 hours of surgery performed by a team led by Dr. Leonard Bailey, whose pioneering infant transplants include the baboon heart implanted in 12-day-old Baby Fae in 1984.

Twenty medical personnel in the operating room, backed up by 80 others, transplanted the heart of a Canadian girl born Monday without a brain.

''We have I think, both mother and baby in good shape,'' Bailey said Friday night.

Bailey said it was unlikely the child would have survived a full day without the operation, but with drugs to help prevent the boy's body from rejecting the new organ, he has a chance to lead a full and normal life.

''Mother and father were able to see Baby Paul immediately following the transplant,'' said Cherie Mathis, heart transplant coordinator at the hospital, referring to Alice and Gordon Holc, of Surrey, a suburb of Vancouver, B.C.

The baby was in a warmer and will be on a respirator for at least two days. The danger of rejection of the new walnut-sized heart is greatest in the first two weeks following surgery, Ms. Mathis said.

Doctors had chilled the newborn and donor heart to 64 degrees Fahrenheit during surgery, bringing body functions to a near halt and plunging the infant into a state of suspended animation.

Once the heart was transplanted, the baby was warmed and the heart began beating.

The heart came from an infant from Orillia, Ontario, whose parents knew early in the pregnancy the child was brainless but went through with the birth for donor purposes, said Leigh-Anne Stradeski, spokesman for University Hospital in London, Ontario.

The child, born on Canada's Thanksgiving Day, was sent to Loma Linda late Thursday, and Paul was delivered by Caesarean section, two weeks premature, to make use of the donor heart.

Previously, the youngest heart recipient was an infant known as Baby Moses, who underwent a transplant performed by Bailey in November 1985. That operation was the world's first successful infant heart transplant.

Baby Moses is doing fine as he nears his second birthday, hospital officials said.

The deformed left ventricle of Paul's heart was only one centimeter wide, instead of the normal two to three centimeters, drastically reducing the volume the heart could pump, doctors said.

His flawed heart was detected through echocardiography, which makes it possible for doctors to diagnose heart malformations in fetuses as early as 14 to 16 weeks after conception.

Echocardiography directs ultrasound waves at the fetus' heart, producing a picture that reveals any structural abnormalities. Sakala said Mrs. Holc had the test because of a history of heart defects in her family.

Bailey, chief of the medical center's pediatric cardiac surgery unit, in October 1984 performed the controversial transplant of a baboon heart into the chest of a 12-day-old girl known as Baby Fae. She survived only 20 days.

Since the surgery on Baby Moses, Bailey has performed seven more such operations on infants younger than 6 months. Four of them are still living.

The four are Baby Eve, Baby Rachel, Baby Jessica and Jesse Dean Sepulveda. The three who died were Trevor Reed, Baby Kari and Baby Victoria.