The mechanism of oxidative damage to the lens through intraocular photochemical generation of superoxide and its derivatization to other oxidants such as singlet oxygen, hydroxyl radical and hydrogen peroxide has been studied. Rat lenses when organ cultured aerobically in TC 199 containing additional amounts of riboflavin were damaged as demonstrated by an inhibition of the uptake of Rb 86 against a concentration gradient. The pump was not affected by light if the culture was conducted in the basal TC 199. However, light was observed to induce significant peroxidative degradation of the tissue lipids even in the basal medium, the degradation being indicated by the formation of malonaldehyde. Both the inhibition of the pump as well as the peroxidative degradation of the tissue lipids, were attenuated considerably by scavengers of superoxide and hydrogen peroxide. In addition, the lipid degradation was prevented by vitamins C and E. The results suggest that the photodynamic injury to the lens cation pump as well as to membrane lipids is incumbent upon an initial generation of superoxide and its derivatization to other oxidants. Thus, the ocular lens is susceptible to oxidative insult and physiological damage through photocatalytic generation of various oxygen radicals. Large concentrations of ascorbic acid in the aqueous humor seems to be able to provide significant protection against such an insult. Thus, this may be one of the functions of high concentration of ascorbic acid in the aqueous humor. The implication of oxidative stress has also been examined in the genesis of cataracts in vivo. Treatment with vitamin E of the Emory mouse led to a decrease in the rate of cataract progression suggesting that at least in some instances an oxidative stress could participate in the formation of cataracts. Oxygen radicals may inflict damage at multifarious biochemical sites. Human lens lipids were also shown to have an absorption maxima at 239 nm indicating their susceptibility to oxidative degradation. In addition the lipid extract has fluorescence similar to that of lipofuscins. The levels of MDA were higher in the brunescent cataracts as compared to that in the nonbrunescent cataracts. The implications of oxidative stress towards the genesis of cataracts in humans is being explored further.