Freshly isolated rat islets were transplanted into the testis of diabetic mice. The intratesticular islet xenografts produced normoglycemia in the diabetic recipients. The survival time of the intratesticular xenografts was significantly longer than intrasplenic, intrahepatic, and renal subcapsular sites of transplantation of xenografts of freshly isolated rat islets. Three of the recipients with intratesticular islet grafts were still normoglycemic at 60 days after transplantation, and orchidectomy resulted in a return to the diabetic state. The findings indicate that the testis provided a more privileged immunologic site than either the spleen, liver, or kidney and the lower temperature of the testis apparently did not affect the function of the islet grafts since the recipients became normoglycemic.