To study the participation of the cerebellum in the sleep-wakefulness cycle, 6 totally cerebellectomized cats and 3 craniectomized sham-operated controls were used. Cerebellectomies were performed by aspiration or by bilateral sections of the cerebellar peduncles. Chronic modifications in the total amount, duration and frequency of episodes of wakefulness (W), drowsiness (D), slow sleep (SS) and paradoxical sleep (PS) were examined. In each group of animals, statistical comparisons between the pre- and postlesion values were made. No changes in the total amount of W, D, SS and PS were observed in the sham-operated group. In all postlesion experiments, cerebellectomized cats presented a statistically significant increase of D (major number of episodes). W, SS and PS modifications only reached statistical significance when considering the whole survival period of the cerebellectomized animals. These consisted in decrease of W and SS (shortening of episodes) and increase of PS (lengthening of episodes). The results, which are discussed in the light of a dual opposing influence of cerebellar cortex and deep cerebellar nuclei on the SWC, indicate an important participation of the cerebellum in the mechanisms of the synchronized EEG together with a regulatory action on fine adjustments of the proportions of the SWC states.