We evaluated the circadian profiles of serum melatonin (MT) and cortisol in 6 patients with Cushing's disease while those of serum MT and GH were evaluated in 8 patients with acromegaly. The control group consisted of 15 healthy subjects in whom MT, cortisol and GH were determined. The presence of a circadian rhythmicity was validated by the cosinor method, while the diurnal and nocturnal amount of MT secretion were expressed in terms of area under the curve. Gross alterations of MT rhythm were not apparent in Cushing's patients. In acromegalics, we observed a blunted day-night oscillation of MT accounted for by a significant increase of its secretion during the day-time period.