Experimental hypertension by the production of renal ischemia1suggests the possibility that the same abnormality underlies the mechanism of human hypertension. Smith and his associates2examined patients with essential hypertension by means of the diodrast clearance test3and reported a reduction in the renal blood flow. However, Chesley and his associates,4employing the same method of determining the renal blood flow, reported that the hypertension found in women suffering from toxemia of pregnancy was not associated with a reduction in renal blood flow, and they were not ableto detect a diminution in renal blood flow in all cases of essential hypertension. Thus there are as yet a relative paucity of information and a lack of complete uniformity in the reports available concerning the relationship of renal blood flow to clinical hypertension. It appeared possible to elucidate this relationship further by means of a study which