Nine days ago Dr. Andrew Morrow of the National Institutes of Health opened the heart of 11-year-old Jane D'Allessio and repaired a serious defect.

Three hours later Dr. Morrow performed the same surgery on the heart of Jane's sister, Cathy, an identical twin with an almost identical defect.

The operations were the first such pair among the more than 5,000 heart operations that have been performed at NIH, and they gave both girls normal blood circulation and a chance to grow normally for the first time in their lives.

Because of the heart defect, each girl measures only 3 feet 11 inches and weighs only 44 pounds. Both now are reported doing "just beautifully" by Morrow, who plans to send them today.

Home is Newark, N.J., from which the D'Allessios' doctor first phoned NIH to arrange for the girl's hospitalization.

At the NIH Clinical Center, a large research hospital at the center of the Bethesda research campus, scores of advanced heart operations are performed. But its surgeons were eager to see what happens when identical twins are born with the same heart defect.

Each twin apparently had a set of defects at birth - a large hole in the wall between the heart's two upper chambers and a tiny defect in the wall between the two lower chambers.

The large hole between the two atria, or upper chambers, was the most serious. When blood returned from the lungs to the heart - carrying a fresh load of oxygen to be pumped through the body - much of the freshed blood spilled back through the hole of the adjoining chamber.

Then the blood was pumped back to the lungs and began moving back and forth heart to lungs without ever reaching the body.

In each girl's heart, the hole between the two upper chambers measured 1.2-by-8 inches and was repaired with a Teflon patch while the blood was temporarily routed through an artificial heart-lung machine. That left the heart nearly blood-free for the surgical work.

Jane's heart also had a small hole between the lower chambers, one so tiny that only a single stitch was required to repair it.

Cathy's heart had a small bulge instead of the tiny hole, indicating that such an opening once may have existed but a sometime had repaired itself.

One preoperative requirement was having enough fresh blood on hand to prime the artificial heart-lung machinery and keep the girls alive in case of bleeding problems. Their blood type is AB positive, one shared by only four of every 100 people.

The Washington Regional Blood Bank of the American Red Cross supplied 21 pints, and the clinical center collected another seven. The Red Cross furnishes about 50 per cent of the blood used by clinical center heart surgeons.

"Actually, both operations were routine," Dr. Morrow said. "Routine, uninteresting, nonheroic, which is exactly the way operations ought to be."

The girls are expected to go home with their father, a warehouse worker.

"I still hurt," Jane reported. Cathy agreed. But now they can chatter happily, just like healthy 11-year-olds.