This study was designed to examine in rats the effects of systemic administration of a potent met-enkephalin analog, D-Ala2-met-enkephalinamide (DALA), on plasma ACTH and corticosterone secretion under basal conditions. Plasma radioimmunoassayable ACTH and corticosterone showed biphasic responses to intraarterial administration of DALA in chronically cannulated, conscious, freely moving, nonstressed animals. Both plasma ACTH and corticosterone increased at 5-10 min and decreased at 45-65 min after the administration of DALA (500 micrograms/kg) compared with either the basal concentrations or the corresponding responses in saline vehicle-injected control rats. Pretreatment with the specific opioid receptor antagonist naltrexone (2 mg/kg, intraarterially) completely prevented the DALA-induced increase in the plasma ACTH concentration and blunted the DALA-induced increase in the plasma corticosterone concentration. Naltrexone also increased plasma concentrations of ACTH and corticosterone above basal levels at a later time in DALA-treated animals. The DALA-induced increase in plasma corticosterone appears to be mediated by ACTH, mice DALA failed to increase plasma corticosterone in either hypophysectomized or dexamethasone-suppressed rats. DALA decreased the adrenocortical responsiveness to ACTH in hypophysectomized, but not in untreated or dexamethasone-suppressed, rats. Naltrexone increased the adrenocortical response to ACTH in DALA-treated rats. These data suggest that endogenous opioid peptides regulate pituitary-adrenocortical secretion under basal conditions by modifying both ACTH secretion and the glucocorticoid response to ACTH.