It is known that the visual loss in severe uveitis is due primarily to retinal tissue damage. In order to test the hypothesis that this damage may result from oxygen free radical-induced peroxidation of retinal membrane lipids, the generation of oxygen metabolites at the site of intraocular inflammation was investigated in an animal model of uveitis induced by retinal S-antigen. The effect of these metabolites on the initiation of retinal damage was characterized by histochemical, biochemical, morphologic, and morphometric methods. Light and electron microscopic studies at the early stage of the inflammation disclosed disorganization, degeneration, and necrosis of the photoreceptors and other retinal cells. Novel histochemical procedures demonstrated formation of superoxide and hydrogen peroxide at the site of uveoretinitis. Chemiluminescence measurements on uveoretinal tissue from these experimental animals revealed generation of superoxide anion and hydroxyl radicals. During the early phase of the uveoretinitis, concomitant with generation of the oxygen metabolites, there was peroxidation of retinal membrane lipids. The peroxidation products consisted of CD, MDA, hydroperoxides, and others. Associated with these changes was a selective depletion of the PUFA 22:6, decrease of which in the retinal composition has been shown to affect visual function. The morphologic and biochemical investigations clearly indicate that oxygen free radicals are generated at the site of uveoretinitis and that the retinal damage is mediated by peroxidation of lipids that are present in the retinal cell membranes. It would thus seem logical that such intraocular inflammation and the resultant retinal damage could be suppressed by antioxidant enzymes and oxygen free radical scavengers. These studies provide for the first time clear indication for developing new therapeutic agents that possess oxygen free radical scavenging properties, for treatment of human uveitis.