Diabetes of various degrees of severity was induced experimentally in rats by different doses of streptozotocin. These animals served as recipients for isolated islets of Langerhans from allogeneic donors. The islets were transplanted to different regions in the organism by paravascular or intravascular injection. As in pancreatectomized rats, the endocrine effect of the islets was only transient and consisted of disappearance of glycosuria, normalization of blood glucose and amelioration of intravenous glucose tolerance tests. When the islets were injected intravascularly (lung, liver) the influence of the transplanted islets was observable over a longer period than after subcutaneous or another paravascular transplantation. As in pancreatectomized animals, the period of survival was markedly prolonged in rats which had received a transplant compared to those which had not. The islets responded to glucose stimulation in vivo with insulin secretion similar to that of control rats, while only a very slight elevation of the low basis levels in streptozotocin-treated rats was observed.