Isogeneic BN rats were made uremic by subtotal renal resections. After different periods of uremia the possibility of an immunodeficiency was evaluated by (WF×BN) F1→BN heterotopic heart transplantations. A prolongation of transplant survival was found that was not related to the duration of uremia. In some rats the uremia was reversed by isogeneic BN→BN kidney transplantation before cardiac grafting. An immediate reversal of the immunodeficiency occurred. Thus an immunosuppressive effect by uremia can be documented, and to some extent quantitated, by heterotopic heart transplantation between rats of different isogeneic str