Neuronal events associated with adaptation of the horizontal vestibulo-ocular reflex (HVOR) induced by sustained vestibular-visual mismatching were investigated in the primate flocculus. The floccular area related to the HVOR (H-zone) was identified by electrical micro-stimulation which induced ipsilaterally directed horizontal eye movement. It was thus found that Purkinje cells in the H-zone consistently changed their simple spike responses to head rotation in parallel with the adaptive HVOR gain change. This was demonstrated by observing the change of simple spike firing of Purkinje cells during adaptation of HVOR either in a population study or an individual study. Since similar changes occurred even after bilateral lesioning of vestibular nuclei had extinguished the HVOR, these changes appear to represent vestibular, but not eye velocity, mossy fiber responsiveness. The complex spike discharge, on the other hand, modulated during vestibular-visual stimulation with a reciprocal pattern to the adaptive changes in the simple spike discharge. These results are consistent with the hypothesis that the flocculus Purkinje cells adaptively control the HVOR through their simple spike activity under influences of retinal error signals conveyed by visual climbing fiber pathways.