The present study attempts to evaluate the possibility of using cryopreserved pancreatic islets in rats. Insulin-releasing activity was studied for the purpose of clarifying the function of cryopreserved islets. Cryopreserved pancreatic islets were also transplanted into the portal veins of recipient rats. Rats recovered from the diabetic state, and the normalized condition was maintained for up to 20 weeks, although 4 weeks were needed before blood and urine glucose reached normal levels after transplantation. Intact B cells were found in the transplanted islet cell masses in the liver of the recipients, but B cells of the recipient's pancreases (streptozotocin-treated rats) showed a marked decrease, as well as degenerative changes.