Under anesthesia with N2O (70%) and halothane (2-4%), Purkinje cell activities were extracellularly recorded in the flocculus and nodulus of immobilized pigmented rabbits. Large field (60 degrees x 60 degrees) optokinetic stimulation (OKS) was delivered to the central visual field of one eye with a constant velocity (0.1-4.0 degrees/s) at 0 degrees, 45 degrees, 90 degrees or 135 degrees to the horizontal plane of the eye. Most of the Purkinje cells in the flocculus and the nodulus showed significant simple spike modulations to OKS delivered to either eye. As a whole, the preferred directions of simple spike responses in the flocculus had the same orientation as those of complex spike responses. However, the preferred directions and amplitudes of modulation of simple spike responses did not necessarily correlate with those of complex spike responses in individual flocculus Purkinje cells. On the other hand, the preferred directions of simple and complex spike responses were not necessarily in the same orientation in the nodulus. The optimum velocity for simple spike responses was in the range 0.1-2.0 degrees/s for Purkinje cells in both the flocculus and the nodulus. The amplitude and time to peak of the simple spike responses of nodulus Purkinje cells were significantly smaller and longer, respectively, than those of flocculus Purkinje cells. In both the flocculus and the nodulus, Purkinje cells whose simple spikes preferred the horizontal orientation (H cells) and the vertical orientation (V cells) showed clustering. In particular, zonal organization was noted in the flocculus. H cells were localized in a dorso-ventral zone in the rostral one third of the flocculus, and V cells were in two distinct zones rostral and caudal to the H cell zone. The locations of H and V cells in the flocculus correspond to the H zone and V zones, respectively, determined on the basis of the preferred directions of complex spike responses to OKS. This indicates that the same subdivisions of the flocculus are supplied with optokinetic signals with the same orientation selectively through both mossy and climbing fibers, and suggest that such subdivisions of the flocculus are functional units which control horizontal and vertical components of optokinetic eye movements. The present results indicate that the flocculus and the nodulus are supplied with distinct optokinetic signals through mossy fibers and play different roles in controlling optokinetic eye movements.