1. The effects of two vasodilator polypeptides, bradykinin and eledoisin, were studied in isolated blood-perfused canine kidneys before and after administration of indomethacin, an inhibitor of prostaglandin synthesis, Bradykinin, but not eledoisin, releases renal prostaglandins. 2. Before administration of indomethacin, bradykinin decreased urinary osmolality and increased free qater clearance, whereas eledoisin did not affect the excretion of solute-free water. After administration of indomethacin, the renal vasodilator action of bradykinin was reduced but the vasodilator action of eledoisin was unaffected. 3. Fractional excretion of sodium was not affected by bradykinin before but was increased after administration of indomethacin. Reduction in glomerular filtration rate contributed to changes in sodium excretion produced by bradykinin and eledoisin. 4. The release of prostaglandins from the kidney by bradykinin amplifies the renal vasodilator action of the kinin and possibly mediates its effect on excretion of solute-free water.