After the portal-embolic transplantation of a low number of isologous islets of Langerhans into streptozotocin-diabetic rats altered liver acini emerge down-stream to the islet grafts. The hepatocytes of these altered liver acini store high amounts of glycogen and/or fat. The proliferation of the hepatocytes of such altered acini and simultaneously their apoptotic elimination were increased strongly, compared with the surrounding unaltered liver acini. After transplantation of a high number of islets, altered liver acini were not observed. The reason for these focal phenomena was supposed to be the persistent systemic hyperglycemic diabetic state which stimulates the beta cells to produce insulin and to secrete it in maximal amounts. This hypothesis has been supported by electron-microscopical examination of the transplanted islets in a former publication, but the proliferative activity of the transplanted islet epithelial cells has not so far been examined. Furthermore, the period of observation has been only three weeks, and it seemed of interest to determine whether the locally increased hepatocytic proliferation and the stimulation of the beta-cells would persist for a longer time. 450-500 (group I, n = 14) or 1500-2000 (group II, n = 8) isologous pancreatic islets were transplanted to adult male Lewis-rats made diabetic by streptozotocin. Group I animals stayed hyperglycemic after transplantation, group II animals became normoglycemic. 5-bromo-2'-desoxyuridine (BrdU) was applied by osmotic minipumps for 6 days before death (6, 13, 21 and 67-77 days after islet transplantation). In group I animals the mitotic index (number of mitotic figures per 1000 hepatocytic nuclei), the apoptotic index (number of apoptotic bodies per 1000 hepatocytic nuclei) and the immunohistochemical-labeling index for BrdU (BrdU-labeled hepatocytic nuclei per 100 hepatocytic nuclei) of these altered acini (A) were compared with the adjacent non-altered acini (NA). The mitotic index in A was 40-fold to 100-fold increased compared with NA (for the whole period of observation). The apoptotic index in A was 6-fold to 20-fold increased compared with NA. The BrdU-LI in A was 20-fold to 80-fold increased compared with NA. In group II animals altered liver acini did not emerge. Furthermore, BrdU-LI of epithelial cells of group I islets were significantly higher compared with group II islets (28.4 vs. 6.3 at day 6, 28.8 vs. 4.2 at day 13, 23.2 vs. 2.9 at day 21, 15.3 vs. 0.1 at days 67 to 77, respectively). It is supposed that two different types of proliferative stimuli were active in this model of low number islet transplantation: 1) The chronic hyperglycemia induces endocrine epithelial cell proliferation in the grafts. 2) The local chronic hyperinsulinism induces glycogen and fat storage, proliferation and apoptotic elimination of the hepatocytes in the liver acini down-stream to the stimulated islets.