Ferrokinetic studies were made in 16 patients before and after kidney transplantation. The Fe clearance decreased in 9 of 11 patients, the plasma iron turnover showed no variation in six patients, while it increased in three and decreased in one, the production of hemoglobin increased significantly after transplantation. The nonnephrectomized patients showed more erythropoiesis after transplantation, manifested by higher hemoglobin level, shorter Fe clearance, and lower serum iron. Uremic patients, including the anephric, had significant responses in erythropoiesis both to stimulation by phlebotomy and to inhibition by blood transfusion. After transplantation, however, the response to phlebotomy was greater and the blockade by blood transfusion smaller, than during uremia.