We measured preretinal oxygen tension over laser-treated and intact retina in normal and diabetic cats. The oxygen tension was significantly higher over the area given panretinal photocoagulation than over the intact retina when the cats breathed 100% oxygen and the same trend was seen when the cats breathed 21% oxygen. There was no difference in preretinal oxygen tension between the normal and diabetic retinas. Light and electron microscopy showed laser damage to the mitochondria-rich photoreceptors in the outer retina as well as the retinal pigment epithelium. The inner retina showed no photocoagulation damage. Because panretinal photocoagulation destroys parts of the outer retina, thus reducing its oxygen consumption and allowing oxygen to diffuse from the choroid into the inner retina, it improves the oxygen supply to the inner retina.