The rate of blood flow through the uvea and the arterio-venous O2 difference across the uveal tract were determined in cats. The normal rate of blood flow was 1.14 ± 0.23 ml/min and the normal a-v O2 difference was 1.02 ± 0.16 volume per cent. The O2 extraction was 8.0 ± 0.6 μl NTP/min. The arterial O2 saturation, Po2, Pco2, and pH were 95.6 ± 0.3 per cent, 93 ± 3 mm Hg, 25.8 ± 1.0 mm Hg, and 7.44 ± 0.01 units, respectively. An artificial rise in eye pressure reduced the rate of uveal blood flow and increased the a-v O2 difference. The O2 extraction was relatively constant at blood flow rates above 0.3–0.5 ml/min. At lower flow rates the O2 extraction decreased in spite of a high venous O2 concentration. Blood collected from the choroidal veins had a slightly higher O2 concentration than blood collected from the intrascleral venous plexus. A reduction in uveal blood flow, produced by a reduction in the mean arterial blood pressure, reduced the O2 content both in blood collected from the choroid and in blood leaving the anterior uvea. The results suggest that at low perfusion pressures there is anoxia in some tissue supplied by the uveal vessels, in spite of a relatively well oxygenated venous blood.