INVESTIGATION of humoral regulation of erythropoiesis has broadened insight into various clinical disorders of red blood cell (RBC) production.1,2Significant observations3have clarified, in part, the pathogenesis of the erythroid failure associated with chronic renal insufficiency. Nonetheless, the complete explanation for this apparently unique abnormality remains uncertain. In the present study, ferrokinetic measurements and plasma erythropoietin assays were undertaken in eight anemic uremic patients before and after the amelioration of the uremic state by peritoneal dialysis. Methods and Results Clinical data are presented in Table 1. Hematocrit readings, hemoglobin levels, reticulocyte counts,4blood urea nitrogen (BUN), and serum creatinine levels were determined prior to and following peritoneal dialysis. Dialysis was performed by the intraperitoneal method5with a commercially available dialysing solution (Peridial) to which 4 mEq of potassium chloride per liter was added. A course of peritoneal dialysis consisted of an exchange of an average of 42