Background/Aims: The number of kidney transplantations performed per year is limited due to the availability of donor organs. One possible solution to the organ shortage is the use of renal xenografts. However, the transplantation of xenografts is complicated by rejection. Methods: It has been postulated that the host immune response might be attenuated following the transplantation of embryonic kidneys (metanephroi) rather than developed (adult kidneys). Transplanted metanephroi become chimeric organs in that their blood supply originates from the host. It is possible to transplant a developing metanephros, without the use of immunosuppression, from one outbred rat to another. Results: Transplanted metanephroi grow, develop, become vascularized, and function in host rats. In contrast, developed adult kidneys transplanted from one rat to another undergo rejection within 7 days after transplantation. Conclusions: These observations suggest that metanephric tissue may be less immunogenic than adult kidney. Transplantation of metanephroi represents a new development that could lead to a novel therapeutic approach to the treatment of chronic renal failure.