Patients with hypopituitarism are predisposed to fasting hypoglycemia and are considered unusually sensitive to insulin-induced acute hypoglycemia. However, whether impaired response of counter-regulatory hormones, such as glucagon, epinephrine (E), and nor-epinephrine (NE) contribute to the susceptibility to acute hypoglycemia in hypopituitary patients has not been systematically evaluated. Therefore, we compared counter-regulatory hormone responses to insulin-induced acute hypoglycemia in 9 patients with hypopituitarism who were off hormone replacement therapy and 13 normal healthy subjects. All subjects received aa prime-continuous intravenous infusion of insulin (0.1 Unit/kg body weight.h) till plasma glucose declined to less than 2.5 mmol/l or occurrence of hypoglycemic symptoms. All normal subjects and 7 out of 9 hypopituitary patients recovered spontaneously from hypoglycemia. Two hypopituitary patients with hypothalamic pathology however needed intravenous glucose, glucagon and hydrocortisone to assist recovery from hypoglycemia. Overall, patients with hypopituitarism showed a slower rate of recovery of plasma glucose after hypoglycemia than normal subjects (0.78 +/- 0.33 mmol/l.h vs. 1.72 +/- 0.15 mmol/l.h, respectively; p = 0.02). The responses of key counter-regulatory hormones, glucagon, E and NE, to hypoglycemia however were essentially similar in both the groups. We conclude that the lack of cortisol (secondary to ACTH deficiency) and GH in hypopituitary patients may be primarily responsible for the slow recovery of plasma glucose after acute hypoglycemia; and plasma glucagon, E, and NE responses are not impaired.