The 24-hour pattern of prolactin, cortisol and growth hormone (GH) secretion, and the diurnal variations of plasma catecholamine levels, were tested in a sample of DSM III schizophrenics and in a control population of patients hospitalized in the same ward and diagnosed as suffering from neurotic disorders. All subjects were acclimated to the ward and kept free from any drug for at least 2 weeks. No difference between schizophrenics and controls was observed with respect to the 24-hour profile of plasma prolactin and cortisol levels. The nocturnal peak of GH secretion was absent in 10 out of the 23 schizophrenics. These patients did not differ from the other schizophrenics with respect to the mean scores on any item of the Comprehensive Psychopathological Rating Scale. The mean plasma noradrenaline levels were significantly higher in schizophrenics than in controls during the wake period, but not during the sleep hours. A consistent decrease of the amine levels during the sleep period was observed in both groups. Since the nocturnal surge of plasma GH concentration has been associated with slow-wave sleep, which has been reported to be reduced in schizophrenic patients, the possibility is considered that this abnormality of sleep pattern can explain the alteration of GH release in schizophrenics. On the other hand, findings concerning plasma noradrenaline are interpreted as an indication that the state of increased autonomic arousal of schizophrenic patients is closely linked to daytime social interaction.