Several experiments in which the effects of clonidine (CLN) have been studied in the rat and in man are summarized. CLN inhibits paradoxical sleep (PS) both in the rat and in man. This inhibition is followed by a rebound, but with a different time course in the two species. Wakefulness (W) is enhanced after some critical doses of CLN, and decreased after higher doses. These effects of CLN suggest that α2-adrenoceptors are involved in the regulation of W and PS