The present study was designed to investigate the effects of prenatal morphine exposure on the hypothalamic-pituitary-adrenal (HPA) axis-regulated stress responses by measuring restraint stress-induced changes in the adrenocorticotropic hormone (ACTH) and corticosterone (CORT) levels. In experiment 1, plasma levels of ACTH and CORT in prenatally morphine-, saline-exposed and control male rats were determined before and at several times after restraint stress. There were no statistically significant differences in plasma ACTH and CORT levels before restraint stress between the groups. However, prenatal morphine exposure dampened the stress-induced increase and spontaneous recovery of ACTH levels after the restraint stress. There were no differences in plasma CORT levels between the three groups either before or at any time after restraint stress. Experiment 2 was designed to investigate the sensitivity of negative feedback of glucocorticoids using the dexamethasone (DEX) suppression test. DEX was administered at different doses (0.001, 0.01, 0.1 and 1.0 mg/kg) and ACTH and CORT plasma levels were measured before and at several times after restraint stress in prenatally morphine- and saline-exposed males. DEX pretreatment eliminated the differences observed in ACTH responses to stress in morphine- and saline-exposed males. DEX pretreatment dose dependently suppressed the restraint stress-induced increased plasma ACTH concentration. In plasma CORT levels, DEX pretreatment dose dependently suppressed the restraint stress-induced increased plasma CORT concentration regardless of prenatal drug exposure. Thus, the present study demonstrates that prenatal morphine exposure alters the ACTH and CORT responses to stress but not the sensitivity of negative feedback of glucocorticoids.