The contents of beta-endorphin (BE), methionine-enkephalin (MEK), and adrenocorticotropic hormone (ACTH) in the hypophysis and hypothalamus of intact 4-to 6-week-old and 16-week-old Wistar rats was studied. The maximum BE concentration was found in the hyphophysis, whereas the maximum MEK and ACTH concentrations were found in the hypothalamus. Aging was followed by a decrease in the concentrations of all above substances, except BE, whose concentration in the hypophysis of the older rat group was markedly higher than in the hypophysis of 4-to 6-week-old animals.