Recently, islet transplantation in patients with type 1 diabetes has had greater success than in the past, but the important question of whether the kinetics of islet secretion are able to accommodate the metabolic demands of special conditions such as exercise remains unanswered. Syngeneic rat islets (4,000 islet equivalents/rat) were transplanted into the liver, kidney, and peritoneal cavity (encapsulated or nonencapsulated) of rats with streptozocin-induced diabetes. Normoglycemic transplanted rats and age-matched controls were subjected to 30 min of moderate exercise on a treadmill 5 weeks after transplantation. Although control rats maintained near normoglycemia during and after exercise, the rats with islet transplants had significantly lower blood glucose levels. For the rats with islets in the liver, increased C-peptide levels were found at 30 min (790 +/- 125 and 1,450 +/- 250 pmol/l at 0 and 30 min, respectively; P < 0.01), whereas a decrease was found in controls and in rats with islets transplanted into the peritoneal cavity or under the kidney capsule. Moreover, increased glucagon levels were found after exercise in the rats with islets transplanted into the liver (62 +/- 6, 165 +/- 29, 155 +/- 27, and 97 +/- 13 pg/ml at 0, 30, 60, and 90 min, respectively; P < 0.05), whereas no changes in glucagon levels were observed in controls. In conclusion, moderate exercise caused hypoglycemia in rats with islet transplants in different sites including liver, kidney, and peritoneal cavity. C-peptide and glucagon responses to exercise were very different in rats with transplanted islets compared with controls. This islet dysfunction led to exercise-induced hypoglycemia.