To examine nephrogenesis in allogeneic grafts, we implanted fetal (day 14 postconception) rat kidneys, which contained only immature tubules, and day 15 to 17 postconception kidneys (with immature tubules and developing glomeruli) into the anterior eye chamber or under the renal capsule of mature rat hosts. Within 9 to 10 days, every graft became richly vascularized, new nephrons were induced to form, and extensive glomerular and tubular cytodifferentiation occurred in all implants. Electron microscopy showed well-formed maturing-stage glomeruli containing erythrocytes, fenestrated endothelial cells, mesangial cells, and fully developed podocytes with foot processes and slit diaphragms. Tubules morphologically corresponding to proximal tubules and containing epithelial cells with tall apical microvilli, as well as distal tubular segments, were also observed. Intravenous injections of antilaminin IgG into hosts resulted in labeling of glomerular basement membranes in grafted kidneys, confirming perfusion of the grafts by host vasculature. Some ultrastructural abnormalities were observed in the grafted kidneys, including the persistence of unfused basement membranes in capillary walls of some maturing-stage glomeruli, and signs of graft rejection were obvious in most samples by 16 days after implantation. Despite these irregularities, however, the ability to establish allogeneic grafts of developing kidneys presents an opportunity to explore glomerular development in a variety of genetic and physiological backgrounds.