The effects of fenfluramine, an indirect serotonin (5-HT) receptor agonist, on sleep and brain indole- and catecholamines were examined in rats. Animals implanted with cerebrocortical and dorsal neck muscle electrodes were continuously monitored by the EEG for 12 hr after i.p. injections of dl-fenfluramine hydrochloride (1, 5 and 10 mg/kg). Fenfluramine produced a dose-dependent suppression of both slow-wave sleep (SWS) and rapid-eye-movement sleep (REMS). Accompanying these effects was a dose-dependent increase in head-shaking, a behavior associated with activation of central 5-HT receptors. The incidence of head-shaking was inversely related to SWS and REMS time. At doses which significantly suppressed sleep (5 and 10 mg/kg), fenfluramine lowered whole brain 5-HT and 5-hydroxyindoleacetic acid concentrations without affecting brain catecholamines. Pretreatment with metergoline (2.5 and 5.0 mg/kg i.p.), a 5-HT receptor antagonist, 1 hr before the administration of fenfluramine (5 mg/kg) blocked the fenfluramine-induced suppression of SWS in a dose-dependent manner and prevented head-shaking behavior, but failed to prevent the suppression of REMS. In contrast, pretreatment with alpha-flupenthixol (0.2 mg/kg i.p.), a dopamine receptor antagonist, had no effect on the suppression of sleep and the stimulation of head-shaking behavior produced by fenfluramine. These data suggest that the suppression of SWS but not of REMS by fenfluramine is mediated by activation of the serotonergic system. The increase in serotonergic activity produced by fenfluramine may result from the drug-induced release of 5-HT with subsequent stimulation of postsynaptic 5-HT receptors. These findings are consistent with our hypothesis that pharmacological stimulation of 5-HT receptors suppresses sleep in the rat.