The 24-h profiles of plasma melatonin and cortisol were evaluated in 7 drug-free male paranoid schizophrenics and in 7 healthy subjects matched to the patients for age, sex, body weight, height and season of testing. Blood samples were obtained at 20.00, 22.00, 24.00, 01.00, 02.00, 06.00, 08.00 and 12.00 h. Light was turned off from 21.00 to 07.00 h. Compared with that of the normal controls, the circadian rhythm of plasma melatonin was absent in paranoid schizophrenics (F7.84 = 7.30, p less than 0.0001; two-way ANOVA with repeated measures) whereas the 24-h profile of plasma cortisol was preserved, although at a slightly higher level (F1.12 = 26.810, p less than 0.0002). The melatonin/cortisol ratio was significantly higher in healthy subjects than in the schizophrenic patients. A functional relationship between disturbances in the melatonin rhythm especially and schizophrenia may be proposed, although the significance of this relationship remains to be elucidated.