The effect of cerebellar lesions upon the pupillary light reflex was examined in anesthetized cats. The pupillary response elicited by a step and sinusoidally modulated light stimulus was used. In acute cerebellectomized cats, the high frequency cutoff of the frequency response of the pupillary reflex was moved to a lower level after cerebellar lesions. Selective lesions were made in the deep cerebellar nuclei and the frequency responses of the pupillary reflex were compared. The fastigial lesions produced the most prominent change in the frequency responses. Electrical stimulation of the deep cerebellar nuclei, especially the fastigial nuclei, induced pupillary dilatation. Thus the cerebellum appears to participate in the control of the pupillary light reflex by improving the frequency responses of the pupil so that it can follow changing light stimuli better, and this control is exerted via the bilateral fastigial nuclei.