Artificial heart recipient Barney B. Clark suffered a series of seizures early this morning, and his condition was downgraded to critical, but by late afternoon he appeared to be "doing quite well," University of Utah officials said today.

Clark remained unconscious under sedation, and doctors were hopeful that he had suffered no permanent brain damage, said Dr. Chase Peterson, university vice president for health sciences.

The seizures involved abnormal bursts of electrical activity in the brain, similar to those experienced by an epileptic.

Peterson told reporters that the cause of Clark's major seizure at dawn is not known. That was followed by several localized seizures in the left leg.

Peterson said brain tests provided some reassurance that development of a major brain hemorrhage or blood clots, which could cut oxygen to Clark's brain and cause brain damage, was "less likely" than doctors had feared earlier in the day. This possibility had "not completely been eliminated," he added.

Instead, Peterson said, it seemed more likely that the seizures may have come from a chemical imbalance developed in Clark's body during recovery from implant surgery last week. This would not do permanent harm, "nothing more than the bad dream we've all had," he said.

Tonight, medical center officials said no briefings on Clark's condition were planned until 11 a.m. EST Wednesday.

This afternoon, Peterson was cautiously optimistic, a contrast to the somber mood earlier in the day when he said that the "potential of this being very serious is significant, very significant.

"This could be catastrophic . . . .We don't know yet if this is just a complication. If it's a metabolic imbalance, that's just a complication. But if it's a hemorrhage or a blood clot, that's a disaster."

He emphasized that such changes are encountered normally but that conducting Clark's case in the glare of publicity tends to accentuate any changes.

Tests conducted this afternoon to measure Clark's brain activity included an electroencephalogram (EEG), which records brain waves, and a computerized axial tomography (CAT) scan to look at brain tissue. The CAT scan appeared "entirely normal," and the EEG showed the brain was "quieting down" after the seizures, Peterson said.

Peterson said that he does not know when the full extent of the problem will be known and that the hospital staff will watch Clark carefully as sedation is decreased because of possible recurrence of seizures. Clark's brain function cannot be ascertained until he has regained consciousness, Peterson said.

In an attempt to correct the chemical imbalances, Clark is being fed a puree of chicken soup and crushed carrots pumped through a tube into his stomach. A tube in the windpipe prevents hazardous vomiting into the lungs, Peterson said.

Since the 61-year-old retired dentist received the world's first permanent mechanical heart last Thursday, he had made steady progress, interrupted only by minor surgery Saturday night to seal a leakage of air from his lungs.

When the surgery began late Wednesday night -- nine hours earlier than planned -- Clark was described as near death. He suffered from an inoperable disease of the heart muscle that had threatened his life when his failing heart began to beat abnormally.

At one point, just before they could attempt the implant, doctors reportedly had to thump on Clark's chest to keep the heart beating. After the surgery, Clark's pale blue pallor, that of a dying man, changed to an encouraging pink, and his vital signs nearly returned to normal.

Following today's early morning seizures, Peterson said Clark had been placed on the critical list.

Peterson said that the seizures were brought under control by medication, including the tranquilizer Valium and an anti-seizure drug called Dilantin, and that Clark was "resting quietly" afterward. The initial seizure involved a generalized contraction of Clark's entire body followed by the seizures in his left leg over a two-hour period.

He said Clark had spent an "excellent" night and, minutes before the seizures, turned to surgeon William DeVries, who implanted the heart, and asked, "How am I doing?"

"You're doing just fine," DeVries replied, according to Peterson.

Detailing possible causes of the seizures, Peterson said they included, in order of seriousness:

* Hemorrhage, or breakage of blood vessels, in the brain. This was potentially the "most serious," he said, and could have caused "major or minor" brain damage by cutting off the oxygen supply where the break occurred.

Arguing against this prospect, he said, were CAT scan findings and Clark's blood pressure, which remained in the normal range. The normal cause of such hemorrhages is hypertension, or elevated blood pressure.

He said hemorrhaging is unlikely to be related to the artificial heart, since Clark's "cardiac function was better than before." It could have been linked to blood-thinning medication Clark was receiving, he said.

* Blood clots, ranging from one to a "shower of little" ones. These can "plug" the blood vessels and cause brain destruction by restricting oxygen supply. This problem has occurred in animals with artificial hearts and could involve a reaction to the plastic surface of the mechanical device, he said.

The CAT scan showed no large clots, although smaller ones could be present, and a separate CAT scan of the artificial heart showed "no evidence of clotting on the surfaces," he said.

* Chemical or metabolic imbalances inside and out of the cells. This disruption could stem from the implant surgery, which included use of a respirator that could have altered levels of gases in the blood, or from "minor kidney failure," he said.

Two kidney function tests indicated that abnormalities had been building since surgery and peaked Monday, he said. A buildup of antibiotics being given Clark to ward off infection could also have contributed to such a disruption, he said.

"Ironically, it's the heart that hasn't been causing us trouble," Peterson said late today.

When surgeon DeVries told Clark's wife, Una Loy, and his children of the serious change in his condition this morning, Peterson described her as "strong as ever" but said there was a "tear or two."

Mrs. Clark later sent a note to be read to the media, saying she and her family were "unprepared" for the publicity surrounding the operation.

She expressed "gratitude to the people of this wonderful country for their thoughts and prayers" but urged that offers to purchase "exclusive rights" to the story be stopped so she can concentrate on her husband's progress.