Defective glucagon secretion during hypoglycemia is characteristic of long-standing type I diabetes. To determine whether this defect can be corrected by successful intrahepatic islet transplantation, we performed studies of hypoglycemia in four nondiabetic patients with chronic pancreatitis who had undergone total pancreatectomy and successful intrahepatic islet autotransplantation, in two type I diabetic recipients of successful intrahepatic islet allotransplantation, and in matched control subjects. We examined 1) whether intrahepatic islet autotransplantation provides glucagon secretion during prolonged periods of hypoglycemia and 2) whether intrahepatic islet allotransplantation in type I diabetic patients and consequent long-term normoglycemia reestablishes native α-cell responses to hypoglycemia. Glucagon secretion was assessed during 3-h hypoglycemic hyperinsulinemic clamp studies. The islet autograft recipients were studied 63 ± 19 months posttransplant, and all were insulin-independent and normoglycemic (HbA(1c), 5.8 ± 0.2%). Neither allograft recipient required exogenous insulin and maintained HbA(1c) levels of 5.7 and 6.4% 30 and 34 months posttransplant, respectively. All recipients were normoglycemic (fasting glucose: autograft recipients, 5.6 ± 0.1 mmol/l; allograft recipient 1, 6.3 mmol/l; allograft recipient 2, 5.8 mmol/l) at the time of study. During hypoglycemia, no increase in glucagon secretion was observed in either the auto- or allotransplant recipients, whereas healthy control subjects and recipients of kidney transplantation had significant increases in glucagon. In contrast, both allo- and autograft recipients had glucagon responses to intravenous arginine. These data uniquely demonstrate that: 1) intrahepatic islet transplant grafts secrete glucagon in response to arginine, but fail to secrete glucagon in response to sustained hypoglycemia; and 2) the restoration of sustained normoglycemia for over 2 years in type I diabetic patients may not reestablish glucagon responses from the native pancreas during hypoglycemia. Transplantation sites other than the liver may be required to achieve normal glucagon secretion from the transplanted islets.