Plasma levels of melatonin and cortisol were measured over a 24-hour period in seven patients with primary obsessive-compulsive disorder (OCD) and seven matched healthy control subjects. In OCD patients, the 24-hour secretion of melatonin was reduced as compared with that in healthy control subjects, whereas its circadian rhythm was preserved. In addition, in OCD patients, the overall secretion of cortisol was higher than that in control subjects, but there was no change in the circadian pattern of cortisol secretion. No correlation was found between clinical parameters and hormone levels.