Abstract: When betaine hydrochloride was given orally 250～1000 mg/kg, the motor activity of mice was reduced 50～81%. The hypnotic effect of pentobarbital was markedly increased after oral administration of betaine hydrochloride at the dosage of 500 mg/kg or 1000 mg/kg. Analgesic effect of betaine was demonstrated in mice by the hot plate method (55℃) and the acetic acid writhing test when given intraperitoneally at the dosage of 540 mg/kg or 900 mg/kg. The analgesic effect of betaine (500 mg/kg) was antagonized by prior administration of atropine (5 mg/kg).Betaine hydrochloride (500 mg/kg) given by gastric intubation was shown to have antipyretic effect in rabbits with ⅳ TAB vaccine fever, but not in normotheric rabbits.