Evaluation of sulfhydryl data derived from the nucleus and cortex of individual human lenses, ranging in age from 40 to 90 years, revealed the following: (a) no significant correlation between the age and the sulfhydryl content of normal and nuclear-cataractous lenses; (b) a significant relationship between the distribution of non-protein-bound sulfhydryl compounds among the lens parts and both age and cataractous state; (c) no significant difference in sulfhydryl data from the cortex between groups of cataractous lenses with increasing nuclear color; and (d) a significant difference in nuclear protein sulfhydryl content between the groups of lenses, varying in cataractous state. If lens parts are separated on the basis of cataract localization, sulfhydryl oxidation in nuclear cataractogenesis seems to be confined to the colored nuclear region. It is suggested that nuclear cataract develops from the inner nucleus towards the cortex of the lens.