1. Renal failure following circulatory shock develops because of per- sistent vasoconstriction which is just sufficient to prevent glomerular filtration. Hypoxia of renal tissue has not been demonstrated in surviving cases. 2. During the low-pressure phase in circulatory shock the remaining blood flow through the medullary regions washes out the osmotic gradient built up by the countercurrent system of Henle's loops, so that a concentrated urine cannot be formed. 3. During recovery from circulatory failure the osmotic gradient of the medullary region can only be built up if sufficient fluid from the glomerular filtrate reaches the countercurrent system. The greater the GF, the faster the gradient is built up. References 1. BoYLAN, J. W. , and E. AssHAUER: Unpublished data. -2. DEETJEN, P. , and K. KRAMER: Pfliigers Arch. Physiol. (G. ) (in press). - 3. KRAMER, K. , and K. ULLRICH: Pfliigers Arch. Physiol. (G. ) 267,251 (1958). -4. KRA- MER, K. , K. THURAU and P. DEETJEN: Pfliigers Arch. Physiol. (G. ) 270, 251 (1960). - 5. KRAMER, K. , and P. DEETJEN: Pfliigers Arch. Physiol. (G. ) 271, 782 (1960). -6. KRAMER, K. , and P. DEETJEN: Unpublished data. - 7. KuHN, W. , and A. RAMEL: Helv. chim. acta 42,628 (1959). - 8. LASSEN, N. A. , 0. MuNcK and J. H. THAYSEN: Acta physiol. Scand. IJ1, 371 (1961). - 9. MUNCK, 0. : Renal Circulation in Acute Renal Failure. Oxford 1958.