This study was intended to investigate the effect of high arterial oxygen tension on the peripheral circulation between normal and hypoxic area by using twelve dogs. For this purpose, acute temporary ischemia was produced at the right hindlimb by an occlusion of the right external iliac artery for 1 and 2 hours, and the left hind-limb was let alone as a control. The peripheral vascular resistance and the arterio-venous difference in oxygen tension between the right and the left hind-limb were measured continuously during 1 hour from immediately after reopening of the blood flow of the right external iliac artery. They were compared between at the condition of breathing room air at 1 ATA and pure oxygen at 2 ATA. The high arterial oxygen tension causes vasoconstriction. When there exists hypoxia in the peripheral tissue, however, it does not act on as a vasoconstrictive factor until the oxygen deficit is improved.