To examine the impact of opiate blockade on glucose counterregulation we performed two hypoglycemic insulin clamp studies with and without naloxone in healthy subjects and well-controlled insulin-dependent (IDDM) patients with defective glucose counterregulation. During both studies plasma glucose fell to 55-60 mg/dl and was then maintained at that level using a variable glucose infusion. In normal subjects, naloxone increased glucose production, thereby reducing the exogenous glucose dose needed to maintain the hypoglycemic plateau. Epinephrine and cortisol responses to hypoglycemia were increased during naloxone plus insulin compared with insulin alone; glucagon responses were unaffected. IDDM patients with suppressed hepatic and hormonal responses to insulin-induced hypoglycemia also demonstrated greater stimulation of glucose production as well as epinephrine, growth hormone, and cortisol release during the naloxone study. In the absence of hypoglycemia, naloxone did not significantly affect glucose production or glucoregulatory hormones. We conclude that opiate blockade augments glucoregulatory responses to insulin-induced hypoglycemia, even in IDDM patients with preexisting defects in glucose counterregulation. This effect is at least in part due to enhanced counterregulatory hormone release during hypoglycemia. Endogenous opiates may modulate hormonal responses during hypoglycemia; their blockade could provide a means of ameliorating defective counterregulation in IDDM patients.