Numerous factors are related to the development of cataract. In particular the effects of free radicals as triggers for the formation of cataracts have recently attracted attention. The oxygen radicals (O2.-) produced by the hypoxanthine-xanthine oxidase system and the hydroxyl radicals (HO.) produced by Fenton reaction were measured using electron spin resonance (ESR) methods. In the present study, we investigated the effects of compounds used as anti-cataract drugs and evaluated their inhibitory effects on radical formation. Pirenoxine and ascorbic acid both caused potent suppression of the radicals.