Transretinal PO2 profiles were recorded with O2-sensitive microelectrodes in the normal retina and in ischemic retinal foci induced by the occlusion of a retinal branch vein with argon laser photocoagulation in anesthetized miniature pigs. In the normal retina there are two PO2 gradients: one from the inner retina and the other from the choroid, both directed toward the middle of the retina. Both PO2 gradients persisted during hyperoxia. Thus, even in hyperoxia, the choroid does not supply the whole thickness of the normal retina with O2. Preretinal and transretinal PO2 measurements in ischemic inner retinal foci showed the existence of two PO2 gradients in steady-state systemic normoxia, as did those in the normal retina. This finding indicates that even in ischemia the choroid does not supply O2 to the inner retina; as a result, tissue hypoxia is maintained. During systemic hyperoxia, the intraretinal PO2 measurements in the ischemic foci showed only one gradient going from the choroid toward the inner retina. This gradient indicates that under these conditions, the choroid can supply O2 to the entire thickness of the ischemic retina. Extending a previously formulated hypothesis, we propose that in the ischemic retina as opposed to the normal retina, hyperoxia does not induce an increase in the O2 consumption of the outer retina. This suggestion could explain the rise in PO2 in the inner ischemic retina during hyperoxia.