In mongrel dogs, the horizontal part of the pancreas was infiltrated with collagenase, cut in pieces, incubated with collagenase, rinsed twice by centrifugation or sedimentation and implanted into the spleen of the same animal. The operations were terminated by the removal of the rest of the pancreas. Of 26 operated dogs, 1 died because of a duodenal perforation, 5 developed severe hyperglycemia without remission, and 20 were long-term normoglycemic survivors followed for up to 10 wk. These 20 animals became spontaneously normoglycemic in the course of the 1st 10 postoperative days. Later, during glucose loading tests, the pattern of blood sugar values was the same in the transplanted animals as in those of a group of non-operated dogs, but the insulin release, although immediate, attained half the control values. The plasma insulin in the splenic vein was more than 7 times higher than in the peripheral circulation. Splenectomies performed in 7 animals were followed by severe hyperglycemia and death. Light microscopy and EM demonstrated the presence of the intact endocrine and exocrine pancreatic tissues in the spleens of all animals investigated. Laborious separations of endocrine from exocrine tissue are not mandatory for ulterior endocrine function, and in an animal larger than rodents it is possible to obtain a diabetes-preventing function after the transplantation of only a part of the gland.