1. Indomethacin inhibited the synthesis of prostaglandins in isolated blood-perfused canine kidneys. 2. Blood flow to the inner cortex was reduced to a greater degree by indomethacin than blood flow to the outer cortex, resulting in redistribution of fractional blood flow to the outer cortex. 3. A prostaglandin, probably PGE2, appears to participate in the regulation intra-renally of medullary vascular resistance and thereby inner cortical blood flow.