In the first series of experiments, preretinal oxygen tensions were measured using microelectrodes in newborn kittens, at 6 +/- 2 days of age, following their placement in an atmosphere of 80 to 90% oxygen for 5 +/- 1 day. The oxygen exposure caused an obliteration of the normally developing retinal vasculature. Preretinal oxygen tensions in the resultant avascular retina were close to zero while those in the vascular retina were near normal. The avascular retina was a "sink" for ocular oxygen. Oxygen breathing resulted in expected increases in preretinal oxygen tension, but, surprisingly, the preretinal oxygen tension decreased with continued oxygen administration. Because the oxygen tension in the avascular retinal area is determined primarily by the choroidal circulation, we speculated that increased oxygen caused a decreased choroidal blood flow. In the second series of experiments, however, choroidal blood flow was measured in the kittens by applying a temperature probe to the sclera and oxygen breathing did not appear to have an effect. These results did not support the initial speculation. It may be that the decrease in preretinal oxygen tension observed with continued oxygen administration resulted from progressive increase in utilization.