Isologous isolated islets of Langerhans were transplanted into the peritoneum and, via the portal vein, into the liver of diabetic rats. In both groups almost normal blood glucose and serum insulin levels were achieved for a period of three months. Glucose tolerance tests were markedly improved. Morphological examination of the transplanted islets and immunohistochemical tests for insulin and glucagon showed the liver to be a more suitable site for islet grafting than the peritoneum.