We studied the effects of aging on the formation of ester forms of riboflavin in the lens of rats. The formation was highest in newborn and 2-week-old rats; a sharp decrease was noted at one month, thereafter it continued to decrease gradually reaching a plateau at 3 months. In general, esterification of riboflavin was higher in female than male rats. We found that a decrease in the esterification of riboflavin with increasing age is a characteristic change in the rat lens. This finding may help to clarify the pathogenesis of senescent change in the lens.