Horizontal eye nystagmus beating towards the stimulated side ('optic' nystagmus) was evoked in 25 guinea pigs by stimulating an optic nerve. The complete ablation of the vestibular nuclear complex abolished this nystagmus. In a second group of 15 animals, the behavior of 63 vestibular neurons identified electrophysiologically was analyzed during eye nystagmus; most of these neurons responded with a tonic increase in discharge rate to stimulation of the optic nerves. Units sensitive to the direction of optic and labyrinthine nystagmus were also identified.