Single unit recordings were performed in the nuclei raphe dorsalis (RD) and raphe magnus (RM) of semi-chronic prepared cats during spontaneous or cryogenically induced sleep. Of the RD neurons 75% showed a discharge pattern which decreased during slow wave sleep and paradoxical sleep (PS); such a decrease in spiking was related to the occurrence of ponto-geniculo-occipital (PGO) waves. to the contrary, 78% of the RM neurons demonstrated a discharge pattern which significantly increased during PS, and especially when PGO waves were present.