The effect of sedatives on the acoustic reflex was studied after administration of diazepam(5mg/kg), phenobarbital(30mg/kg), thipental sodium(25mg/kg) and chloral hydrate(50mg/kg) in rabbits. Acoustic reflex threshold(ART) was raised after administration of these drugs. The crossed ART shift were greater than the uncrossed in all groups, particularly with significant ART shift in groups administered chloral hydrate and thiopental sodium. In all groups the duration of recovery from the induced ART shift was found to be prolonged in the crossed ART than in the uncrossed. These results suggested that the effect of sedative on ART was principally due to depressive action of drugs on the central nervous system, though the direct action of drugs on the middle ear muscles could not be excluded. The difference of the patterns of ART changes between the administered sedatives resulted from the differences of the action of the drugs on the central nervous system and the middle ear muscles.