Inbred Lewis rats made diabetic by streptozotocin were transplanted with 430-870 islets from syngenic donors via the portal vein. Though fasting plasma glucose was normal, the tolerance to glucose given intraperitoneally (IPGTT) or orally (OGTT) remained impaired one month after transplantation. The higher the number of islets, however, the better the glucose tolerance. The results of IPGTT and OGTT in each animal correlated. Fasting plasma insulin levels were within the normal range suggesting that the action of insulin on the liver and its degradation by the liver is impaired due to the limited number of hepatocytes subjected to high insulin concentrations. Eight months after transplantation, the plasma insulin and glucose responses to OGTT were still unchanged while those of the less physiological IPGTT were worse. Thus, the functional capacity of the transplanted endocrine tissue does not appear to change with time.