The effects of a single injection of parachlorophenylalanine (PCPA, 300 mg/kg i.p.) on sleep, motor activity and consummatory behavior were investigated in unrestrained rats which were continuously recorded with telemetric techniques on two control days and six drug days. Slow wave sleep (SWS) was defined as the non-REM sleep (NREMS) fraction with a low predominant EEG frequency. In the 24 h following PCPA administration, motor activity and food intake were reduced and sleep was increased. SWS was massively enhanced, while REM sleep (REMS) was depressed. The initial phase of sedation was followed by a phase of partial insomnia lasting 1-2 days. SWS and REMS were particularly depressed. A rebound phenomenon was observed at the end of recovery period when some of the SWS and REMS values exceeded the control level. The administration of tryptophan (Trp, 150 mg/kg i.p.) 28 h after PCPA pretreatment, causing a significant rise in the brain serotonin (5-HT) concentration, produced a temporary increase in SWS and REMS, and a reduction of motor activity. The experiments show that the depression of SWS and REMS, and the hyperactivity 1-2 days after PCPA administration, are a consequence of the reduced 5-HT level, whereas the involvement of serotonergic mechanisms in the initial sedative phase and in the recovery phase is less clear. The persistence of the daily distribution of sleep and activity, and of the specific pattern of SWS and REMS, indicates that the circadian sleep organization is little affected by 5-HT depletion.