Long-term survival of islet allografts was achieved with an experimental model in the rat by the transplantation of islet/kidney composite grafts. Morphologic studies of these grafts incorporating immunocytochemistry have been made. Ther was minimal evidence of rejection of the transplanted islet tissue, which was composed almost entirely of insulin-containing cells. There were a few glucagon-containing cells, but pancreatic polypeptide-labeled and somatostatin-labeled cells were rarely seen. The endocrine cells were in different proportions and had no discernible relationship to one another, in contrast to those in nontransplanted islets. For successful clinical transplantation, it may be necessary to maintain normal relationships between endocrine cells.