Oxygen-sensitive microelectrodes were used to measure the intraretinal oxygen distribution in the rat during both air and oxygen breathing. During air breathing the average PO2 profile obtained showed a value of 19 +/- 2.5 mmHg (n = 36) at the internal limiting membrane (ILM). As the penetration depth increased, a minimal PO2 of 8.2 +/- 2.4 mmHg was attained at 50% of the retinal depth, from which point the PO2 increased to a peak of 33 +/- 3.3 mmHg at 100% of the retinal depth at Bruch's membrane. During oxygen ventilation, all intraretinal oxygen tensions were increased and the profile became almost monotonic, increasing from 36.1 +/- 3.3 mmHg (n = 9) at the ILM to 134.3 +/- 6.7 mmHg at 100% of the retinal depth. These results are qualitatively similar to those found in the cat, and they demonstrate the importance of oxygen delivery by both the retinal and the choroidal circulation during normoxia, as well as the inability of vascular autoregulation to maintain a constant retinal PO2 during hyperoxic ventilation.