Studies were performed to determine the effect of injecting repeated doses of erythropoietin (Ep) on the red cell volume of chronically uraemic rats and on that of non-uraemic sham operated ones. After 13 doses of Ep (5 u/dose), started either 5 or 21 d after removal of five-sixths of the renal mass, the increase in the red cell volume of uraemic rats was as great as that of non-uraemic ones. The significance of these results is discussed.