Erythrokinetic studies on two anephric and asplenic patients are described. The patients were nephrectomized in preparation for renal homotransplantation and maintained on hemodialysis for six and eight months. Results of these studies indicate that despite absence of kidneys and spleen, and intrinsic base line erythropoietic activity remains, with a relatively constant and persistent reticulocyte production and radioiron turnover. Autonomy of this erythropoietic activity was apparent by lack of responsiveness to induced anemia and polycythemia. In the second patient a reticulocytosis and increased iron turnover occurred five months postnephrectomy. This was accompanied by significant plasma titers of erythropoietin-like substance. A renin-like enzyme was also found in the plasma of both patients. These observations suggest the existence of an autonomous base line erythropoietic activity of bone marrow and a possible functional relationship between inappropriate secretion of extrarenal "erythropoietin" and extrarenal "renin."