The heterogeneous signal content of floccular Purkinje cell responses to optokinetic stimuli was analyzed in alert rabbits by means of selective lesions to brainstem pathways. Extracellular spike activities of Purkinje cells were recorded from rostral areas of the flocculus where local electrical stimulation elicited abduction of the ipsilateral eye. Chronic unilateral destruction of the nucleus reticularis tegmenti pontis, interrupting the visual mossy fiber afferent pathway to the flocculus, reduced the gain of the optokinetic eye movement (OKR) to one-third of the control. Concomitantly, simple spike responses of Purkinje cells to optokinetic stimuli were reduced to less than one-third of the control values. Severance of the visual climbing fiber afferent pathway by rostral inferior olivary lesions reduced the OKR gain little, and decreased the simple spike responses of the Purkinje cells only slightly. Bilateral lesions of the rostral half of the medial vestibular nucleus and rostro-ventral part of the lateral vestibular nucleus, which reduced the eye velocity in the OKR to less than one-third of the control value, did not induce any appreciable change in the simple spike responses of the Purkinje cells. It is concluded that visual mossy fiber signals are the most dominant factor which determines Purkinje cell responses to optokinetic stimuli, while visual climbing fiber signals and eye velocity mossy fiber signals make only subsidiary contributions.