The aim of this study was to investigate if the blood flow of transplanted islets is affected by the implantation organ or regulated by the grafted islets themselves. For this purpose adult rats were partially depancreatized and islets were isolated from the excised pancreatic tissue, maintained in tissue culture for 7 days, and subsequently 500 islets were implanted into the same animal beneath the renal capsule. Four weeks after transplantation, the rats were given an i.v. injection of either saline or furosemide (7.5 mg/kg body weight). Fifteen minutes later the blood perfusion of the whole left kidney and its islet grafts were measured separately with a microsphere technique. Also, the blood flow values of the pancreatic remnant were determined. Rats that were partially pancreatectomized and transplanted showed a decreased whole pancreatic blood flow in the pancreatic remnant after furosemide injection, whereas the blood flow to the islets was not significantly affected. Furosemide also increased the blood flow to the kidney but had no effect on the blood perfusion of the pancreatic islets grafted into the same kidney. These results suggest that the blood flow of an islet graft does not necessarily change in concert with the blood flow of the implantation organ. This may reflect differences in the blood flow regulation between the grafted islets and the implantation organ. In addition, it could be that the vascular system developing in the transplanted islets originates from either the islets or the kidney capsule, rather than from the kidney parenchyma. This may account for a different blood flow regulation.