(1) The bilateral destruction of the region of the dorsal norepinephrine bundle in the isthmus or in the mesencephalon induces a significant increase of both paradoxical sleep (PS) (up to 400%) and of slow-wave sleep (SWS). The increase of PS lasts for 4-5 days while the increase of SWS lasts at least 8-10 days. The same phenomenon is observed after total hypophysectomy. (2) control lesions effectuated in the isthmus outside the area of the dorsal norepinephrine bundle do not affect significantly the slee-walking cycle. (3) pretreatment with p-chlorophenylalanine suppresses the increase of sleep if the lesion is effectuated at the time of maximum insomnia, while a significant increase of PS is still observed if the lesion is made at the time of the recovery of sleep. (4) the mechanisms of this hypersomnia are discussed in light of the biochemical results showing an increase of the biosynthesis of serotonin in the rostral raphe system. It is postulated that some neurons of the dorsal norepinephrine bundle might tonically control the activity of the raphe system.