Electron micrographs of avian retinal pigment epithelium (RPE) reveal a basal zone of abundant mitochrondria, approximately 15% of which present ring-shaped profiles. We infer that these ring profiles represent cross sections of“donut-shaped”bodies, randomly deployed in the RPE mitochondrial zone. The per cent of all RPE mitochondria which are in the ring form increases with age, and is further increased in chicks reared under continuous light, an environmental condition leading also to light-induced avian glaucoma (LIAG). We calculate that a given mitochondrion, on assuming the ring form, would more than double its surface area. Thus the RPE mitochondria of chicks developing LIAG may present to the surrounding cytoplasm a total surface area more than fourfold greater than in normal chicks.