Mixtures of carbon dioxide and oxygen are commonly used in the treatment of central retinal artery obstruction to improve retinal oxygenation. While oxygen alone causes retinal vasoconstriction, it is thought that the carbon dioxide balances this effect, even causing a net vasodilatation. To test this hypothesis, normal, healthy volunteers were given 100% oxygen, a mixture of 95% oxygen and 5% carbon dioxide, and a mixture of 5% carbon dioxide in air to breathe. The caliber of large, fluorescein-filled retinal arteries and veins was then measured using computer processing of digitized television images. The marked decrease in arterial and venous caliber caused by 100% oxygen was not reversed by the subsequent addition of 5% carbon dioxide. Moreover, 5% carbon dioxide in air did not cause substantial vasodilatation of the retinal vasculature. The efficacy of adding 5% carbon dioxide to oxygen to treat retinal vascular obstructive diseases is questioned.