Based on electrophysiological, neurochemical, genetic and neuropharmacological approaches it is currently accepted that serotonin (5-HT) functions to promote waking (W) and to inhibit rapid-eye movement sleep (REMS). The serotonin-containing neurons of the dorsal raphe nucleus (DRN) provide part of the serotonergic innervation of the telencephalon, diencephalon, mesencephalon and rhombencephalon of laboratory animals and man. The DRN has been subdivided into several clusters on the basis of differences in cellular morphology, expression of other neurotransmitters and afferent and efferent connections. These differences among subpopulations of 5-HT neurons may have important implications for neural mechanisms underlying 5-HT modulation of sleep and waking. The DRN contains 5-HT and non-5-HT neurons. The latter express a variety of substances including dopamine, γ-aminobutyric acid (GABA) and glutamate. In addition, nitric oxide and a number of neuropeptides have been characterized in the DRN. Available evidence tends to indicate that non-5-HT cells contribute to the regulation of the activity of 5-HT neurons during the sleep–wake cycle through local circuits and/or their mediation of the effects of afferent inputs. Mutant mice that do not express 5-HT_1A or 5-HT_1B receptor exhibit greater amounts of REMS than their wild-type couterparts. 5-HT_2A and 5-HT_2C receptor knockout mice show a significant increase of W and a reduction of slow wave sleep that is related, at least in part, to the increased release of norepinephrine and dopamine. A normal circadian sleep pattern is observed in 5-HT₇ receptor knockout mice; however, the mutants spend less time in REMS. Local microinjection of 5-HT_1B, 5-HT_2A/2C, 5-HT₃ and 5-HT₇ receptor agonists into the DRN selectively suppresses REMS in the rat. In contrast, microinjection of 5-HT_1A receptor agonists promotes REMS. Similarly, local administration of the melanin-concentrating hormone or the GABA_A receptor agonist muscimol produces an increase of REMS in the rat. Presently, there are no data on the effect of local infusion into the DRN of noradrenergic, dopaminergic, histaminergic, orexinergic and cholinergic agonists on sleep variables in laboratory animals.