More than 300 patients at the National Institutes of Health research hospital in Bethesda were returned to their rooms by late yesterday afternoon, one day after a fire at the hospital forced their evacuation.

Montgomery County fire inspectors said the full extent of damage and the cause of the fire-which broke out on the ninth floor of the NIH Clinical Center-will not be determined until the end of the week.

According to NIH spokeswoman Doreen Mead, most of the damage occurred in the west wing of the ninth floor of the building, located at 9000 Rockville Pike.

Two NIH policemen and four fire-fighters who suffered burns and smoke inhalation during the blaze were released yesterday from Suburban Hospital.

An infant who was transported to Holy Cross Hospital suffering mild smoke inhalation was reported in good condition yesterday. Mead said the baby would be returned to the hospital today.

After being evacuated and spending several hours in surrounding parking lots, most of the patients returned to their rooms Saturday. About 50 patients evacuated from the eighth floor of the building spent the night at various clinics on the NIH campus.

They returned to the Clinical Center at 3 p.m. yesterday after NIH personnel spent most of the day mopping, stripping beds and clearing smoke-filled rooms.