• Disturbances were studied in the nocturnal circadian pattern of serum melatonin and plasma cortisol levels in 33 female patients with an eating disorder (anorexia nervosa, n=11; bulimia nervosa, n=12; or both, n =10) and in ten female control subjects of comparable age. Blood samples were obtained hourly from 8 PM to 6 AM under controlled darkness. Serum melatonin levels in all patient groups were initially similar to those of control subjects. When patients were divided according to depression status, those with concurrent major depression had significantly lower nocturnal melatonin values than the nondepressed group. Weight did not appear to influence melatonin levels. In contrast, all patient groups had significantly higher nocturnal levels of plasma cortisol than control subjects, and neither weight nor depression separated patient groups on profiles of plasma cortisol.