Alterations of the hypothalamo-pituitary-adrenocortical (HPA) system are well-known phenomena in human aging as well as under stressful conditions and in psychiatric disorders. Among the various neuroendocrine function tests developed so far, the combined dexamethasone (DEX)/corticotropin-releasing hormone (CRH) test, in which DEX-pretreated subjects receive a single dose of CRH, has proved to be the most sensitive measure of subtle changes in HPA system regulation. To further explore the mechanisms underlying these neuroendocrine abnormalities in an animal model, a combined DEX/CRH test was established in young male Wistar rats. Five days before the experiment, the jugular vein was catheterized under halothane anesthesia for subsequent drug infusion and blood sampling. DEX (30 micrograms/kg) administered at 12.00 h, during the diurnal trough, suppressed the diurnal increase in circulating corticotropin (ACTH) and corticosterone between 18.00 and 20.00 h, during the acrophase. Subsequent CRH (50 ng/kg) infused at 20.00 h provoked a minimal escape from DEX suppression, indicated by a slight increase in ACTH and corticosterone secretion. Therefore, the combination of 30 micrograms/kg DEX given at 12.00 h followed by pituitary-adrenal system stimulation with 50 ng/kg CRH at 20.00 h was defined as the standard DEX/CRH test procedure and was then used in young (3-6 months) and aged male Wistar rats (20-24 months). After DEX treatment, basal ACTH levels between 18.00 and 20.00 h were significantly higher in aged than in young rats (77.6 +/- 23.2 vs. 19.9 +/- 0.9 pg/ml; p < 0.01), indicating resistance of the HPA system to the suppressive effect of DEX. In addition, the ACTH response to subsequent CRH was significantly higher in aged than in young animals (area under the concentration time curve: 3,670 +/- 2,230 vs. 294 +/- 112; p < 0.05). Thus, the HPA system appeared to be profoundly dysregulated in aged male Wistar rats. The elevated basal ACTH levels reflect glucocorticoid nonsuppression, suggesting negative feedback impairment. This is further supported by the elevated ACTH response to a subsequent CRH challenge, which, in addition, may indicate changes in the endogenous synergistic mechanisms of CRH with other corticotropic factors, for instance vasopressin. In summary, the DEX/ CRH test revealed HPA system alterations in aging and can be applied in future studies to further explore the mechanisms underlying the neuroendocrine disturbances during (psycho) pathological states.