Immunoreactive beta-endorphin was determined in the plasma of 37 elderly subjects (73 +/- 7 years of age), in 3 young or young adult subjects without adrenal disease, in 4 women with clinical adrenogenital syndrome, in 1 man with Cushing's disease. Immunoreactive beta-endorphin in plasma was measured by radioimmunoassay in six samples of each subject collected at 4-hour intervals over a 24-hour span. The observation of a circadian rhythm in circulating immunoreactive beta-endorphin with highest values during the late night and early morning hours is extended to elderly subjects in the 8th decade. The beta-endorphin plasma concentrations in the few clinically healthy young adult subjects studied fell within the same range. The circadian variation of the group of elderly subjects was used as reference in the clinical evaluation of plasma beta-endorphin concentrations in patients with pituitary-adrenocortical disorders. The need for circadian rhythm qualified reference values is shown by the observation of the circadian variations of circulating immunoreactive beta-endorphins in the patients with adrenogenital syndrome and Cushing's disease in whom abnormalities in the concentration of circulating beta-endorphin were found at certain circadian stages but not at others.