We investigated the effects of aging on the regulation of hypothalamic-pituitary-adrenal function and hippocampal steroid receptors in a series of in vivo and in vitro studies conducted in healthy intact 2-, 8-, 18-, and 24-month-old male Fischer 344/N rats. Basal plasma ACTH levels were similar among age groups, and basal plasma corticosterone levels showed a significant aging-associated decline. Two i.v. doses (2 and 20 micrograms/kg BW) of rat CRF elicited significantly greater and delayed ACTH and greater corticosterone responses in older rats, consistent with the pattern encountered in hypothalamic CRF deficiency. In contrast, the i.v. injection of a muscarinic agonist, arecoline, elicited similar ACTH and corticosterone responses in all age groups. An i.v. injection of ACTH-(1-24) evoked lower corticosterone responses in the older (18- and 24-month-old) than in the younger (2- and 8-month-old) groups of rats, consistent with an impairment of hypothalamic-pituitary-adrenal axis function in older animals. Steady state mRNA levels of mineralocorticoid and glucocorticoid receptors were significantly decreased in the hippocampus of the 8-, 18-, and 24-month-old rats, compatible with maturational, rather than senescent, changes. CRF mRNA levels in the paraventricular nucleus of the hypothalamus, CRF content, and in vitro secretion by whole explanted hypothalami were progressively and significantly reduced with age, whereas the steady state levels of arginine vasopressin mRNA were significantly increased with age. Steady state levels of POMC mRNA were decreased, and ACTH content and in vitro secretion by corticotrophs were increased with age in the anterior pituitary. We conclude that male Fischer 344/N rats show a progressive hypothalamic CRH deficiency with advancing age, which appears to be associated with elevated production of arginine vasopressin in the hypothalamus.