Single unit spike activities of Purkinje cells in the cerebellar flocculus were examined during sustained horizontal sinusoidal oscillation (0.33 Hz, 2.5 degrees peak-to-peak) of a striped screen around an alert pigmented rabbit. The floccular area specifically related to horizontal reflex eye movement (H-zone) was identified by means of local stimulation that induced abduction of the ipsilateral eye. In control states, simple spike discharge of most of the H-zone Purkinje cells was enhanced by backward screen movement and depressed by forward screen movement, while complex spike discharge was modulated reciprocally. After one-hour sustained oscillation of the screen, the gain of horizontal optokinetic eye movement response (HOKR) increased by 0.16 on average. Correspondingly, simple spike modulation in most of H-zone Purkinje cells tested significantly increased in amplitude, while complex spike modulation tended to decrease. No such systematic changes were observed in other Purkinje cells. These results are consistent with the hypothesis that the floccular H-zone Purkinje cells adaptively control the optokinetic eye movement through modification of the visual mossy fiber responsiveness under the influence of the retinal error signals conveyed by the visual climbing pathway.