Since very little is known about the impact of aging on adrenocortical function in female rats, it was of interest to compare the age changes in the circulating levels of adrenocorticotropin (ACTH) and corticosterone in male and female Sprague-Dawley rats, and correlate these changes with those in plasma prolactin (Prl), which is known to stimulate adrenal steroid secretion. Hormones were measured in young (3–4 months) and old (24–26 months) male rats as well as in young (3–4 months), old (25 months) and senescent (33–35 months) female rats. Sequential plasma samples were obtained from chronically cannulated animals every 30 min from 11:30 a.m. to 4:30 p.m. ACTH was measured in trunk serum – obtained between 11:30 a.m. and 1:30 p.m. – whereas corticosterone and prolactin were measured in plasma using specific RIAs for the corresponding hormones. No age changes were detected in serum ACTH in either sex. The integrated values of plasma corticosterone did not change with age in males but decreased significantly (p < 0.05) in old and senescent as compared to young females. Plasma corticosterone was higher in young females than in young males but this sex-related difference disappeared with age. Plasma Prl increased significantly with age in both male (p < 0.05) and female (p < 0.001) rats but showed no significant correlation with corticosterone levels. The present results suggest that ACTH secretion does not show major age- or sex-related changes. Our corticosterone data are compatible with the idea that gonadal aging has a significant impact on plasma glucocorticoid regulation in female rats but n