The mean 24-hour secretion and circadian rhythm of melatonin were studied in 12 female subjects with anorexia nervosa (AN), 13 massively obese (OB) women, and 9 normal weight healthy volunteers to investigate the relationship between type of feeding behavior and hormonal secretory pattern. Blood samples for melatonin were drawn every 4 hours from 0400 h to 2400 h and every 2 hours from 2400 h to 0400 h. Mean 24-hour melatonin secretion was significantly higher in AN than in OB patients and controls. Melatonin circadian rhythms were disrupted in 8 of the 12 AN patients and in 9 of the 13 OB subjects, with phase-advanced nocturnal rises, abnormal diurnal peaks, or no nocturnal rises. The population mean cosinor analysis validated the existence of a significant circadian rhythm of the hormone in AN but not in OB subjects. No significant correlation between mean 24-hour secretion or type of circadian alterations and degree of weight deficit or excess was observed. The circadian alterations of melatonin in AN and OB may be linked to impaired secretory tonus of noradrenalin in the central nervous system, possibly unrelated to feeding patterns.