We evaluated the effect of previous experimental hypoglycemia on counterregulatory responses to hypoglycemia in 13 IDDM patients. These patients had defects in counterregulatory responses to hypoglycemia compared with 7 nondiabetic control subjects. Plasma EPI and glucagon responses to hypoglycemia in IDDM patients were ∼60% of levels in nondiabetic subjects (P < 0.02 and P < 0.001, respectively). Hepatic glucose output ([3-³H]glucose) was reduced by ∼60% of normal (P < 0.005), and the glucose infusion rate required to maintain plasma glucose was correspondingly greater in people with IDDM (P < 0.001). With a modified glucose clamp (plasma insulin ∼330 pM), the diabetic subjects underwent two sequential 120-min periods of hypoglycemia (∼3.0 mM) with an intervening 60-min euglycemic recovery period. In the IDDM patients, there were 30–50% decreases in plasma GH (P < 0.005) and cortisol (P < 0.001) responses during the second hypoglycemic period compared with the first. In addition, glucose output, already defective compared with that in nondiabetic subjects, was further reduced by 33% (P = 0.03) during the second period of experimental hypoglycemia. There was no effect of repeated hypoglycemia on the responses of plasma glucagon, EPI, or NE, though plasma EPI was correlated directly with glucose output (P < 0.001) and inversely with glucose uptake (P < 0.05). There was no correlation between the rise in glucose output during hypoglycemia and antecedent glycemic control as measured by HbA₁. We conclude that in IDDM patients with preexisting defects in counterregulatory responses to hypoglycemia, recurrent, mild hypoglycemia is associated with additional reductions in pituitary-adrenocortical hormonal secretion and further impairment of hepatic glucose production.