Abstract Electrical stimulation (30 min, pulse 0.5 ms, 150 μA, 20 Hz) of the antero-dorsal part of the nucleus raphe dorsalis (n.RD) induces in the basal hypothalamus (n. arcuate and surrounding areas) a large axonal release of 5-hydroxyindole compounds (5-OH Les; about +300%) measured by means of voltammetric technique. During such a release, evidence for a direct detection of serotonin (5-HT) is reported. The above stimulation induces also marked aversive behaviors together with prolonged polygraphic arousal. Three hours later, a significant paradoxical sleep (PS) rebound occurs. Both the 5-OH Les release and the PS rebound observed after n.RD stimulation are suppressed by a p-chlorophenylalanine (PCPA) pretreatment of the animals. The 5-HT-hypogenic factor dependence of the PS rebound observed after n.RD stimulation is discussed.