2. Diodrast clearance loses its value as a measure of renal plasma flow (a) during severe or prolonged hypotension, and (b) immediately after restoration of arterial pressure by infusion after a first stage of hypotension. In the former case (a) it is decreased in consequence of decreased renal extraction of diodrast from blood which itself is presumably the result of distribution of renal blood flow through nephrons whose vascular resistance is low. (b) The disparity after transfusion is due to excretion of diodrast accumulated in the kidney tubules and, presumably, in the interstitial fluid during hypotension. The presence of diodrast and inulin in the renal interstitial fluid explains the addition of these substances to renal venous blood observed in some of these experiments. The shifting equilibrium of diodrast between renal plasma and interstitial fluid may explain some of the instances of low diodrast extraction observed in these experiments during transfusion after hypotension, and may explain also the incomplete renal extraction of diodrast under normal conditions.

3. Dogs with denervated kidneys respond to blood transfusion and restoration of arterial pressure by a disproportionately slow and incomplete return towards normal of renal clearance and, presumably, of renal blood flow. On the basis of these facts it is suggested that high spinal anesthesia may interfere with recovery of renal circulation in cases of shock treated by transfusion.

4. Profound or prolonged and repeated hypotension due to bleeding decreases the ability of normal and denervated kidneys of intact and anesthetized dogs to respond to transfusion and the restoration of arterial pressure by proportionately increased clearance and plasma flow, apparently because of renal vasoconstriction due to the release of humorally circulating vasoconstrictor substances.