Diabetes is associated with increased basal hypothalamo-pituitary-adrenal (HPA) activity and impaired stress responsiveness. Previously, we demonstrated that the HPA response to hypoglycemia is significantly impaired in diabetic rats. In this study, we wished to 1) differentiate between the effects of hyperinsulinemia and those of hypoglycemia per se and 2) establish whether diabetes lowers peak stress responses. Normal and streptozotocin-diabetic rats were subjected to hyperinsulinemic-euglycemic glucose clamps to evaluate central and peripheral responses. These were compared with peak ACTH and corticosterone responses to restraint and hypoglycemia. Hyperinsulinemia increased corticotrophin-releasing hormone and vasopressin mRNA, and plasma ACTH and corticosterone in normal and diabetic rats. In normal animals, insulin-induced activation of ACTH and corticosterone was lower compared with responses during either restraint or hypoglycemia. In contrast, ACTH and corticosterone activation in diabetic rats was similar in all three stressors. Pituitary-adrenal axis activation in diabetic animals was also much lower compared with normal controls. The response to hyperinsulinemia (euglycemia) was associated with increases in glucocorticoid receptor mRNA in the anterior pituitary and paraventricular nucleus. Hippocampal mineralocorticoid receptor (MR) mRNA expression was increased in normal animals, but not in diabetic animals. We speculate that the ability to appropriately match the HPA response to the potency of a stressor is related to the ability to alter hippocampal MR expression. In diabetes, this ability is impaired and hence, maximal HPA activation is greatly diminished. This is a novel observation that may have important implications in the treatment of impaired counterregulatory mechanisms in human diabetes.