The effect of parachlorophenylalanine (PCPA 300 mg/kg i.p.) on several physiological and behavioral parameters was investigated with telemetric methods in the unrestrained rat. Body temperature did not change with the exception of an immediate and short-lasting decrease after drug injection. Food and water intake were maximally depressed on the day following drug administration, and recovered gradually during the subsequent days, drinking more rapidly than feeding. Click-evoked potentials recorded from the auditory cortex and inferior colliculus maintained their typical waveforms during synchronized and dcsynchronized sleep indicating that PCPA does not produce a qualitative change of the sleep stages. A short-lasting increase of the potentials was observed after drug injection. PCPA exerted profound changes on motor activity. The activity during the light periods was significantly increased. However, motor behavior was altered more in its temporal pattern than in intensity, especially during the dark periods. The circadian rhythms of feeding, drinking and motor activity were attenuated. Since the time-course of these changes corresponds to that known for serotonin depletion in the brain, serotoninergic neuronal mechanisms may play a major role in the organization of behavioral rhythms.