That serotonin (5-HT) plays a determinant role in sleep was first suggested by the well-known PCPA-5HTP (p.chlorophenylalanine-5-hydroxytryptophan) paradigm. This involvement, however, is paradoxical since localized cooling of the nucleus raphe dorsalis (n.RD) is sleep inducing, and unitary activity of 5-HT neurons decreases during slow wave sleep (SWS) and paradoxical sleep (PS). Furthermore, on the basis of voltammetric 5-hydroxyindole (5-OHles) measurements, it appears that 5-HT could be released throughout the sleep/wake cycle according to two different modalities: by the axonal nerve endings during the waking state (W) and by the dendrites and/or the soma during sleep. The axonal release of 5-HT might participate in sleep preparation by stimulating the synthesis of hypnogenic factors within target structures like the basal hypothalamus (BH). When such a release is increased by an immobilization stress (IS) or electrical stimulation of the n.RD, a sleep rebound is induced. The somato-dendritic release of 5-HT might be primarily responsible through an auto-inhibitory process for the decrease and abolition of the 5-HT neuronal unitary activity as well as for the reduction of the axonal release of 5-HT; both phenomena being constantly observed during sleep. Finally, the hypnogenic factors might initiate and maintain sleep by influencing the n.RD sleep gating mechanisms either through the somato-dendritic release of 5-HT, or directly.