Exposure of rat lens to fluorescent daylight (150 ft candles) under tissue culture conditions led to a substantial lipid peroxidation as evidenced by the formation of malonaldehyde (MDA). MDA content of lenses incubated overnight in presence of such light was approximately sixfold of that in the control lenses cultured in the dark. These cultures were maintained in physiological medium resembling aqueous humor which does not contain any additional photoactive component. Thus, the lens in its physiological surroundings is susceptible to photoperoxidation by light of wavelengths which freely penetrate the eye. Photoperoxidation could be thwarted by superoxide dismutase, catalase, and ascorbate, suggesting that the observed peroxidative degradation is initiated by photocatalytic generation of superoxide and its subsequent derivation to other potent oxidants. These studies provide for the first time suggestive evidence that senile cataract development may in part be linked to the in vivo photochemical generation of superoxide and other potent oxidants in the aqueous humor and lens derived from the ambient oxygen and light; and ascorbate which is maintained at high levels in this fluid by virtue of its active transport from plasma, is physiologically important in preventing the deleterious action of these potent oxidants. The studies thus indicate for the first time the possibilities of a hitherto unrecognized role of ascorbate against cataracts and other age-, light- and oxygen-dependent ocular abnormalities, In addition, the study re-emphasizes the role of tissue catalase and superoxide dismutase in the prevention of photoperoxidative damages to the tissue.