White House press secretary James Brady, who was shot in the head in the assassination attempt on President Reagan last March, suffered a serious brain seizure yesterday morning and was still being kept unconscious under an anesthetic last night to try to prevent further damage.

He lost consciousness and went into convulsions at around 8 a.m. The grand mal seizure -- which is the same as a seizure in severe epilepsy -- was brought under medical control by anesthesia and drug treatment by George Washington University doctors.

At 9 p.m., a medical report said he was still being kept asleep under barbiturate anesthesia in the hospital's post-anesthesia recovery room under the watch of his chief neurosurgeon, Dr. Arthur Kobrine.

Such a seizure often occurs in a patient with the kind of penetrating brain injury Brady suffered. On the one hand, it could be a passing event with no permanent consequences if he is safely restored to consciousness and the seizure is not repeated. This often is the case.

On the other hand, seizures could cause more brain damage or death if repeated again and again and if anti-convulsive drugs prove ineffective.

Brady's course on such drugs has been difficult. He was placed on one such drug as a routine preventive measure after his surgery, but became allergic to it.

He became allergic to a second anti-convulsive drug, too, and was on still a third one when he was stricken yesterday.

George Washington University doctors said Brady "had as grand mal seizure in his room immediately after breakfast" and "a subsequent CT scan [a sophisticated X-ray of the brain] show no apparent cause," only "continued healing" of his March 30 gunshot wound.

The George Washington University Medical Center issued a late afternoon medical bulletin that said Brady's "vital signs are normal."

David Gergen, White House communications director, told reporters that chief of staff James Baker conferred by telephone with Dr. Kobrine, who described the episode as "disappointing."

But Korbine, who performed Brady's original brain surgery, also said "almost as surprising as it was when it happened was the fact that it didn't happen before," Gergen said. The neurosurgeon also reported the convulsion "has been contained with anti-seizure medicine."

Grand mal seizures are characterized by rapid, sometimes violent, muscle twitching. The seizure often lasts for several minutes, leaving the victim open to potentially harmful injuries. Victims often bite their tongues as they clench their jaws.

Brady's doctors would answer a few further questions. They would not say whether he spontaneously regained consciousness before treatment or how widespread his convulsions were, describing these matters as "confidential." Both are important in assessing the seriousness of his attack.

His doctors also called the use of an anesthetic "routine treatment" but neurosurgeons not connected with Brady's case said an anesthetic is generally used to halt a continuous, otherwise uncontrollable episode before it exhausts the patient and perhaps causes more damage.

Brady had much of the forepart of his brain either shot away or damaged last March 30, and doctors called his survival a "near miracle."

When he survived, however, they became optimistic about his chances for a good future recovery and a return to work. They brought him through two complications: leakages of air and fluid that threatened to cause a brain infection and a blood clot that reached his lungs.

But Kobrine has pointed out repeatedly that anyone with so serious a brain injury risks a number of complications for a year or longer.