Renal hemodynamies were evaluated in 30 intact anesthetized dogs at 2.36 and 3.6 atmospheres absolute (ATA) of oxygen pressure using a dye-dilution method to measure blood flow. In 15 animals studied at 2.36 ATA the asterial pO2exceeded 1400 mm Hg. Renal blood flow, renal blood volumes, mean arterial pressure, stroke volume, renal fraction of cardiac output and the renal arteriovenous O2difference did not change significantly, while renal resistance and total systemic resistance increased 10 and 28%, respectively. In 15 animals studied at 3.6 ATA the arterial pO2exceeded 2400 mm Hg; significant mean decrease occurred in renal blood flow with no significant change in the renal fraction of cardiac output. Renal and systemic resistance increased 46 and 63%, respectively. The renal arteriovenous O2difference increased as renal blood flow fell; therefore, renal oxygen uptake was unchanged. These studies suggest a selective effect of hyperoxygenation on the renal and systemic circulation with a higher increase in the systemic resistance than renal resistance.