This study examined whether the pharmacologic manipulation of catecholaminergic systems would affect the hypnotic action of flunitrazepam in rats. Flunitrazepam, a potent hypnotic, was used alone or in combination with alpha-methyl paratyrosine (alpha MPT), an inhibitor of the synthesis of catecholamines, and clonidine (CLN), an alpha 2-adrenoceptor agonist. Flunitrazepam significantly increased the amount of slow-wave sleep and the latency of paradoxical sleep (PS) and decreased the amount of PS. Administration of flunitrazepam to alpha MPT-treated rats significantly increased the number of sleep cycles and PS episodes as compared with flunitrazepam alone. Clonidine decreased total sleep time and significantly decreased PS. The association of flunitrazepam with CLN induced a decrease in PS and waking as compared with flunitrazepam alone. The possible involvement of noradrenergic mechanisms in modulating the effect of flunitrazepam on the rat sleep-waking cycle is proposed.