Canine pancreases underwent cold storage in Collins' solution (CS) or in silica gel-filtered plasma (SGF) for 3, 6, and 24 hours before collagenase digestion and autotransplantation of dispersed pancreatic tissue that contained islet cells to the spleens of totally pancreatectomized dogs. The functional outcome in these animals was compared with animals that received intrasplenically autotransplanted pancreatic islet cell preparations from unstored pancreases. Normoglycemia was restored in 15 of 20 (75%) dogs that received freshly prepared islets. Two of five dogs (40%) whose pancreases were stored in CS for 3 hours became normoglycemic, as did 2 of 5 dogs (40%) whose pancreases were stored for 3 hours in SGF. When 6-hour storage was used only 1 of 5 dogs (20%) whose pancreases were stored in CS and 1 of 5 dogs (20%) in whom SGF was used for storage became normoglycemic. Normoglycemia occurred in only 2 of 16 dogs (12.5%) whose pancreases were stored for 24 hours (one in SGF and one in CS). The results indicate that islet cell preparation should be performed immediately after pancreatectomy. The failure thus far of clinical islet allotransplantation may be due, at least in part, to functional deterioration during preservation of the donor pancrease before isolation of islet cells.