Electric stimulation of single ampullary nerves in alert cats and monkeys produced rapid stereotyped head movements. These head movements were similar in the cat and monkey and appeared to be in a plane parallel to the plane of the canal whose nerve was stimulated. Body and limb responses also were evoked. They were more pronounced in the cat than in the monkey and consisted of varying degrees of ipsilateral forelimb extension, contralateral forelimb flexion, curvature of the trunk, and contralateral circling. By stimulating canals singly or in combination head movements could be produced in any spatial plane. These findings demonstrate the specific nature of the reflex head and postural movements produced by the semicircular canals in these animals.