No information is available on the rate of blood flow in transplanted islets. In this study, adult rats were partially depancreatized, and islets from the excised pancreas were then isolated, maintained for 7 d in tissue culture, and subsequently transplanted back to the animal, beneath the renal capsule. Some rats were rendered diabetic with streptozotocin before transplantation. A month after transplantation the blood flow of the grafts was measured by a microsphere technique. Autotransplantation to streptozotocin-diabetic rats of approximately 500 islets did not revert the hyperglycemia, and the blood flow of these grafts was approximately 25% of that in the normoglycemic-transplanted rats. However, in insulin-treated diabetic rats the blood flow of the pancreatic graft was similar to that in the normoglycemic rats. The present results suggest that the blood flow in transplanted islets is markedly diminished by hyperglycemia and that this can be enhanced by insulin administration.