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We determined the effect of perfusion pressure on the distribution of blood flow within the renal cortex of the anesthetized dog. Total blood flow to the denervated kidney was measured by an electromagnetic flowmeter and perfusion pressure was manipulated over the autoregulatory range (136 to 81 mm Hg) by an aortic clamp. Distribution of cortical blood flow was determined by the radioactive microsphere technique. Tissue perfusion rates of four cortical zones of equal thickness differed significantly from each other at normal arterial pressure. In sequence from superficial to deep, perfusion of the cortex zones averaged 4.18, 6.80, 3.07, and 1.68 ml. g−1. min−1. Aortic constriction significantly reduced perfusion of the outer cortical zone and augmented perfusion of the inner two cortex zones. Twelve percent of total renal blood flow was redistributed by pressure reduction. Atropine, 1 mg/kg, had no effect on cortical perfusion rates at either pressure. The influence of microsphere diameter and density on cortex perfusion rates was systematically studied and found to be minor. This observation implies that the microsphere method is valid for measuring distribution of blood flow within the renal cortex. We conclude that zones of the renal cortex are perfused at different rates and respond differently to changes in arterial pressure.