More than 300 patients at the research hospital of the National Institutes of Health in Bethesda were evacuated yesterday, some in their beds or in wheelchairs, after fire broke out in the building, authorities reported.

At least two firefighters were injured in combating the blaze, which broke out about 3 p.m. on the ninth floor of the 13-story red brick hospital that offers new and experimental treatments.

The blaze was confined to the ninth floor where damage was described as extensive. No injuries to patients were reported and most returned without incident to their rooms on other floors by late last night, authorities reported.

A two-month-old infant with heart disease remained at Holy Cross Hospital, where he was taken during the evacuation, as a precaution.

Electrically powered equipment for assisting breathing or for monitoring heart function was disconnected while the patients were moved, then plugged in again on the first floor, where many patients were brought.

Respirators, used to assist breathing, were operated by hand by nurses during the move between floors.

Four patients with weak natural defenses against disease remained in special isolation quarters on the second floor. The smoke did not threaten them, according to Dr. Mortimer B. Lipsett, director of the clinical center.

A fifth patient with similar problems was removed from his protective surroundings on the 13th floor. He spent about a half hour wearing a mask and wrapped in a sterile blanket, Lipsett said.

The patient, also brought to the first floor, according to the official, was later placed in a space suit.

It was an "exciting time," Lipsett said, adding that it was the first full evacuation ever held at the clinical center.

"Fortunately," he said, "we didn't have any major problems."

About 200 nurses were on duty at the time, he said, along with physicians and support personnel, and they "all pitched in."

No cause for the blaze has been determined. It began near a pediatric ward in a lounge area that contained a television set and some chairs, according to Lipsett.

A nurse recalled smelling what she thought might have been smoke, but reminded herself that "we always get strange odors here."

Then, she said, she realized it was smoke, and heard one of about a half dozen patients in the ward asking "What do we do?"

She said she began closing doors to seal off the area as other nurses began carrying the patients from their rooms, racing down the corridor to the stairwell just ahead of the billowing smoke.

As the smoke began spreading to other floors, authorities said, they began to evacuate the 12 upper floors of the building. The operation took fewer than 30 minutes, they said. Although the hospital has about 540 beds, weekend passes cut yesterday's patient roster to about 300 persons.

While some more seriously ill patients were brought to first floor treatment areas, other patients sat in the auditorium or went outside to wait in the bright spring sunshine. CAPTION: Picture, Firemen assist hospital officials in moving patients during blaze. None of the patients was injured. By Steven H. Eisen for The Washington Post