Summary1. Polycythemic mice and hypophysectomized rats were used as assay animals to measure the erythropoietic stimulating activity of synthetic angiotensin II (A-S) and of angiotensin II made from a crude extract of the hog kidney (A-CE). 2. No increase in erythropoietic activity, as measured by the red blood cell incorporation of Fe59, was found following injection of these substances. The assay animals were responsive to a concentrated sheep plasma erythropoietin. 3. These results suggest that the reninangiotensin system does not encompass erythropoietin but do not rule out the possibility that a second material is released by the juxtaglomerular cells.