X-ray photography was employed to analyse the postural syndromes following unilateral global and selective lesions of the vestibular apparatus in guinea pigs. Following hemilabyrinthectomy, head rotation in the horizontal plane resulted mainly from rotation of the cervical vertebrae whereas lateral head tilt was due to the rotation of thoracic vertebrae about the longitudinal axis. These results support our previous conclusion that because of both resting posture and intrinsic biomechanical constraints, the number of degrees of freedom of the cervical column is reduced and the cervical vertebrae are functionally specialized. Selective lesions of the vestibular apparatus have aided in determining the origin of the different components of the hemilabyrinthectomy syndrome: rotation of thoracic vertebrae was caused by unilateral lesion of the otolithic system whereas rotation of the head in the horizontal plane resulted from unilateral lesion of the horizontal semicircular canal system. We conclude that the functional segmentation of the cervical column corresponds to a differential distribution of vestibular afferents.