The effects of increased arterial partial oxygen tensions (po2) on retinal circulation of the young kitten were investigated by observing changes in the velocity of fluorescein wave fronts in the major arterial branches of the retina. Changes in velocity were correlated with simultaneous carotid arterial po2 determinations. These studies on the larger retinal vessels are the first of a series aimed at determining levels of arterial po2 that can be sustained without damage to any components of the retinal vascular bed.