Plasma melatonin circadian profiles were investigated in a group of 4 patients with anorexia nervosa and 4 healthy regularly cycling women. There were no differences in the mean age of both groups, whereas the anorexia nervosa patients had lower mean body weight (37.8 +/- 2.0 vs 57.0 +/- 4.9 kg) and body mass index (13.9 +/- 1.1 vs 20.8 +/- 2.0). Samples were collected every 2 h and plasma melatonin was measured by using a RIA with an iodinated tracer. Anorexia nervosa patients exhibited higher diurnal (60.7 +/- 1.8 vs 25.4 +/- 1.72 pmol/l, P less than 0.02) and nocturnal (419.2 +/- 37.4 vs 108.0 +/- 33.6 pmol/l), P less than 0.001) mean plasma melatonin concentrations. There were no differences in the time peak for nocturnal melatonin secretion in both groups, detected at 02.00 h. In anorexia nervosa, the melatonin circadian profile paralleled that observed in the control group, indicating that the increased melatonin values for anorexia nervosa were probably due to an enhanced secretory pineal function rather than an impaired melatonin metabolism. These results suggest a participation of the pineal gland in the pathophysiology of anorexia nervosa.