Morphological (light microscopical, immunohistological and electron microscopical) findings in the recipient liver of rats with streptozotocin-induced diabetes, obtained 9 months after intraportal injection of neonatal isologous pancreatic islets, are described and their significance discussed. The results support the assumption of active ingrowth of nonmyelinated nerve fibers into the islet isografts. The hepatocytes surrounding the islet isografts contain--obviously owing to the influence of unusually high and locally variable concentrations of insulin--a focally increased number of enlarged mitochondria, abundant glycogen and a smaller amount of neutral fat droplets. Furthermore, hepatocytes and cells looking like hepatocytes (hepatocyte-like cells) with typically structured cytoplasmic beta (insulin) granules were found bordering the islet isografts. These results could be interpreted as an expression of artificial or nonartificial fusion of beta cells with hepatocytes, i.e. formation of hybrid cells ("in vivo hybridization"). Alternatively, they might reflect insulin uptake and storage in the hepatocytes. In addition, these findings suggest that contact between neonatal islet tissue and liver tissue could be a trigger for the in vivo transformation (modulation) of differentiated cells of similar embryonic development in the adult organism.