The influence of aging on the steroid secretory capacity of the adrenal gland was evaluated by comparing data on young (age 20 to 21 years) with elderly (age 77 to 86 years) healthy male subjects. After the administration of ACTH-Z (1 mg, im) during the treatment of dexamethasone (1 mg/day, for 2 days), blood samples were taken at time 0, 1, 2, 3, 6, 12 and 24 h. The mean basal levels of pregnenolone (P5), 17-hydroxypregnenolone (170HP5), dehydroepiandrosterone (DHEA), progesterone (P4), 17-hydroxyprogesterone (170HP4), androstenedione (A-dione) and aldosterone (Ald) gradually decreased with advance in age. Dexamethasone administration to the elderly men produced no significant fall in plasma P4 and Ald. Plasma ACTH levels after ACTH-Z administration were significantly higher in the elderly men than the comparable levels in the young men. The apparent half-life of ACTH-Z in plasma was prolonged in the elderly men. For 3 hours after ACTH-Z injection, the responses of all plasma steroids, such as P5, 170HP5, DHEA, P4, 170HP4, A-dione, Ald and cortisol (F), were significantly lower in the elderly men. When the 24-hour secretion rates of steroid hormones were compared by delta area, which indicated the increased area for 24 hours after ACTH-Z administration, the secretion rate of F showed no significant difference between the two groups, but that of DHEA was significantly low in the elderly men. The 24-hour secretion rates of P5 and P4 were not impaired and that of 170HP4 was significantly high in the elderly men. These results indicate that the steroidogenic response to ACTH decreases with aging, and that, in the elderly men, an apparent decrease in C17,20 lyase efficiency may be related in part to the decreased secretion of adrenal androgens.