Current concepts of the pathogenesis of anemia in uremic animals are derived mainly from the results of studies performed either in vitro or in bilaterally nephrectomized animals. These data may not be applicable to the situation which exists in more chronically uremic animals. In 1932, Chanutin and Ferris showed that removal of five-sixths of the renal mass caused rats to become uremic and to remain so for a prolonged period of time. Rats made uremic in this manner were utilized as models for studying the pathogenesis of the anemia of uremia. Removeal of five-sixths of the renal mass of rats caused their BUNs to rise to over 100 mg/100 ml and to remain at this level for over 3 wk. The hematocrits of these uremic rats fell from 42% to approximately 30% in 3 wk. Erythropoietin (Ep) production immediately fell to a barely detectable level postoperatively and did not increase significantly in 3 wk, although the renal remnant hypertrophied. Extrarenal Ep production also remained at a low level and did not increase during the 3-wk observation period. The response of plethoric uremic rats to 2 units of Ep was as great (in some experiments greater) as that of sham- operated ones. A surprising finding was that plethoric uremic rats, injected with saline rather than with Ep, incorporated more 59Fe into their red blood cells than did sham-operated ones. This finding suggested that in uremic rats erythropoiesis was less markedly suppressed by plethora than it was in non-uremic rats.