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After control selective left renal arteriograms were obtained in dogs, an acute hypotensive state was induced by the rapid removal of blood until the mean aortic pressure reached approximately 50 mm Hg. Subsequently, mean pressures as low as 30 mm Hg were attained. Arteriograms during hypotension revealed narrowing and irregularity of vessels. There was also an alteration in the distribution of renal blood flow: the outer renal cortex was no longer perfused as in control studies, while perfusion of the inner cortex and outer medullary regions was sustained. An apparent increase in total renal vascular resistance was denoted by narrowing of the main and segmental renal vessels, and protracted arterial circulation time. After retransfusion to baseline pressures, follow-up arteriograms from one to 23 days revealed well-defined renal vascular alterations, with poor cortical filling which gradually improved. Segmental vessel irregularities and cortical defects were observed on the serial studies.