Presidential press secretary James S. Brady regained consciousness but remained in critical condition in the intensive care unit at George Washington University Hospital yesterday, after undergoing a 6 1/2-hour operation in which doctors removed a large portion of the right frontal lobe of his brain.

Despite extensive destruction of tissue on the right side of his brain and some damage to the left frontal lobe, Brady's condition improved dramatically in the hours after surgery. He was awake, able to see, and able to move his right arm and leg when asked to, according to doctors at the hospital.

Considering the severity of Brady's brain injuries, one of the doctors called his progress "extraordinary."

"Everybody's very encouraged," said Brady's old boss, Sen. William Roth (R-Del.), after speaking with Brady's wife and one of his doctors at the hospital. "Things are going better than we had hoped for or were prepared for."

However, one of the doctors involved in the case said it was too early to be certain whether his improvement would continue. "We're very guardedly, cautiously optimistic," he said, "But the problem is that within the first 24 to 36 hours after this kind of ordeal things can change. By tomorrow or the next day, if they haven't changed, I'd feel a little more confortable. . . ."

The bullet that struck Brady entered his head above the left eye, passing through the tip of the left frontal lobe of his brain. It then crossed to the right side of the brain, where it did far greater damage -- causing severe bleeding in the right fronted lobe, according to doctors at the hospital.

One doctor said the initial CAT-scan -- a cross-sectional X-ray of the brain done Monday as soon as Brady reached the emergency room -- looked like "a disaster." He also said that before surgery Brady's doctors feared that expansion of the right half of the brain from blood and swelling might cause fatal pressure on the brain stem, maintains breathing and awareness.

A doctor involved in the case, said that the team of neurosurgeons, headed by Dr. Arthur In the case said that the team of neursurgeons, headed by Dr. Arthur I. Kobrine, removed "a lot of brain tissue" from the right frontal lobe. They also took out the bullet, which had lodged in the back portion of the right side of the brain. A much smaller amount of tissue destroyed by the bullet at the tip of the left frontal lobe was also removed, the doctor said.

Doctors do not operate in every case to remove damaged brain tissue. In this case, however, emergency surgery was done to remove blood and to decrease the pressure on undamaged portions of the brain.

The long-term effects on Brady of the loss of brain tissue from the frontal lobes are not yet known.

"We believe he is going to live," Dr Dennis O'Leary, dean for clinical affairs at George Washington University Medical School, said in a statement yesterday morning. But he added, "In patients like this, spectrum of possible outcomes is very, very wide. . . . We have no idea where he is going to end up."

The major function of the frontal lobes of the brain is the control of voluntary muscular movement of the body, with the left frontal lobe affecting movement of the right half of the body, and right frontal lobe controlling movement of the left half of the body.

In addition, a portion of one of the frontal lobes -- most often the left -- contains the brain's speech center. In Brady, who is right-handed, the speech center would be located on the left side, and thus may have been unaffected by his wound, according to Dr. Harvey Ammerman, a clinical professor of neurosurgery at George Washington University Medical School.

A doctor at the hospital said it was impossible to test Brady's ability to speak until a breathing tube is removed. The tube has been passed through his mouth into the windpipe.

The frontal lobes also contain areas that subtly influence personality. Patients whose lobes on both sides have been largely destroyed -- such as mental patients who have undergone prefrontal lobotomies -- show permanent personality changes, including apathy and loss of drive, according to Dr. Alfred J. Luessenhop, chief of neurosurgery at Georgetown University Medical School.

However, permanent changes may not occur in Brady, who apparently suffered little destruction of the left frontal lobe, Luessenhop said.

Initially, he said, patients who have lost most of one frontal lobe often show profound apathy or indifference. "They're inappropriate in moods, and they're not spontaneous," he said. "They'll answer questions with whole sentences but not volunteer anything."

Within six to 12 months, however, the personality functions of the lost frontal lobe can be taken over by the one on the opposite side -- "really remarkably completely in some people," he said. The amount of recovery varies from patient to patient, and in Brady will depend on how much damage was done to the left frontal lobe, he said.

Doctors said Brady received two intravenous medications, mannitol and a drug related to the hormone hydrocortisone, to reduce swelling of the brain. Ammerman said it would also be routine procedure to administer antibiotics to prevent a brain infection, one possible complication of his surgery.

Luessenhop said the fact that Brady was reportedly awake and responsive yesterday, and that his pupils were small, equal and reacting to light, were very hopeful signs. "He'll be a little worse probably for a few days," because temporary swelling may develop from the surgery, he said.