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We have reported earlier that in alert cats the cerebellar flocculus contains many Purkinje cells that respond to pitch (vertical) rotation and that the majority of them receive excitation from the contralateral posterior canal. To understand the role of the flocculus in oculomotor function, I first examined the simple-spike activity of floccular Purkinje cells responding to sinusoidal pitch rotation and vertical optokinetic stimuli, and then injected the GABA agonist muscimol into the flocculus to examine oculomotor deficits. The great majority of Purkinje cells responded to upward pitch rotation and downward optokinetic stimuli, with their response phases near the resultant eye velocity. Following muscimol injection into the up-pitch areas (1 microgram), the down vestibulo-ocular reflex (VOR) induced by up-pitch was depressed at low (not high) stimulus frequencies. Down optokinetic responses were virtually abolished. The cats failed to maintain downward gaze position, and postsaccadic exponential centripetal drift appeared with the mean time constant of 0.7 s. Ocular torsion also appeared on the eye ipsilateral to the injection side in 2 of 3 cats examined. These results indicate the involvement of the flocculus in down and torsional slow eye movements.