Discharges of Purkinje cells were recorded from the vermis, lobules VI and VII, of a monkey trained to track a visual target. When the monkey tracked a sinusoidally oscillating target, cellular activity changes in phase with the velocity signal of the eye movement. When the monkey fixated a stationary point, almost identical modulation in activity occurred, reflecting the velocity signal of the motion of the retinal image of the target. The data suggest that the vermis participates in the control of smooth pursuit eye movements by providing the oculomotor system with the actual target velocity information which is the sum of eye velocity and retinal image velocity signals.