Oxygen consumption rate (QO2) was determined in the outer and inner halves of the cat retina in dark and light adaptation. Double-barreled oxygen microelectrodes were used to measure oxygen tension (PO2) across the retina of anesthetized cats while the single retinal artery supplying that area was occluded. During the measurements of these PO2 profiles, the cats were ventilated with 100% O2. Retinal PO2 profiles were fitted to a diffusion model, and inner and outer retinal QO2s were determined from the fitted parameters. A four-layer model, in which two layers consumed oxygen, fitted the data well. One consuming layer corresponded to the photoreceptor inner segments, as in previous studies, and a single region of uniform consumption was used to describe the profile in the inner half of the retina. Under dark-adapted conditions, outer and inner retinal QO2 were 3.9+/-2.8 and 3.5+/-1.7 ml O2/(100 g. min)(mean+/-SD; 9 cats), respectively. With steady illumination, outer retinal (photoreceptor) QO2 decreased to 1.4+/-0.9 ml O2/(100 g. min), but inner retinal QO2 remained unchanged at 3.7+/-1.5 ml O2/(100 g. min)(5 cats). The total QO2 of the inner retina was found to be the same as that of the dark-adapted outer retina. Oxygen use was distributed uniformly throughout the inner retina but was confined to the photoreceptor inner segments, which occupied approximately 20% of the thickness of the outer retina. Steady illumination had no effect on inner retinal QO2.