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, one died because of a duodenal perforation, five developed severe hyperglycaemia without remission, and 20 were long-term normoglycaemic survivors followed for up to 10 weeks. These 20 animals became spontaneously normoglycaemic in the course of the first 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 seven times higher than in the peripheral circulation. Splenectomies performed in seven animals were followed by severe hyperglycaemia and death. Light and electron microscopy demonstrated the presence of the intact endocrine and exocrine pancreatic tissues in the spleens of all animals investigated. It is concluded that laborious separations of endocrine from exocrine tissue are not mandatory for ulterior endocrine function, and that 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.