Recent studies showed enhanced regeneration of pancreatic islets in some circumstances. The purpose of our study was to investigate the proliferate potential of rat pancreatic islet cells in allogeneic grafts. Adult Lewis female rats and WAG male rats served as recipients and donors, respectively. Diabetes was induced by single intravenous (IV) injection of streptozotocin producing diabetes as confirmed by nonfasting plasma glucose >300 mg% on 3 consecutive days. Islet rejection was considered complete when glycemia exceeded 250 mg% and was confirmed by histopathological examination. To obtain long survival of allogeneic islets a tolerance-inducing method used allogeneic UV-B irradiated bone marrow transplantation into nonlethally selectively cytoreducted recipients with a donor-type splenocyte infusion followed by cyclophosphamide 200 mg/kg bw. Endocrine cell proliferation was assessed morphometrically using double immunostaining for pKi-67 and insulin or glucagon. Double immunolabelling, propidium iodide staining, and TUNEL assay were used to identify both proliferating and apoptotic cells. The rise of glycemia >350 mg/dL after graftectomy in euglycemic recipients was correlated with immunohistological examination, showing that the euglycemia was due to properly functioning pancreatic islet allotransplants. The immunohistochemical examination confirmed the presence of endocrine beta and alpha cells. In comparison with normal pancreas which showed 0.4 +/- 0.12%, pKi-67-positive cells, long-surviving grafts had a significantly higher proliferation capacity (5.61 +/- 0.94%; P <.001). In contrast, rejected grafts/control groups did not show significantly enhanced proliferation (0.73 +/- 0.19%), and had endocrine cells undergoing apoptosis. The incidence of apoptosis in endocrine cells within long-surviving graft appeared to be extremely low. In conclusion, the growth and death of endocrine cells in allogeneic grafts differ between accepted and rejected cases. The level of proliferation in the graft at day 150 was significantly higher compared with normal pancreatic beta cells.