A reproducible model of a diabetic state was produced after resecting more than 90% of pancreatic tissue in Wistar strain male rat (outbred and inbred group). Of a total of 10 animals of outbred group, 6 animals illustrated a blood glucose level over 250 mg/dl in an earlier stage after an injection of 1500 islets into the portal vein. However, in 4 of them blood glucose levels were maintained within normal limit until 4 months. On the other hand, 5 inbred rats in which 500-600 islets were injected demonstrated reversal of diabetes within 1 week after the injection and the effect of the transplantation lasted until about 6 months. A morphometric analysis using 1190 serial histological sections of the control rat pancreas showed that the islet distribution accorded satisfactorily with Weibull or Gamma distribution. The estimated values of size and number of the islet were 0.0155 mm and 131.2/mm3, respectively, while volume ratio of splenic and gastric segments or parabiliary and duodenal segments was estimated to be 1.45 or 0.98%, respectively. The mean size of the isolated islets under dissecting microscope reached 0.080 mm and 500 of them were equivalent to about 10% volume fraction in a whole control pancreas. In the inbred group a transplanted animal contained about 17% of islet volume as compared with control animal.