Aldose reductase (AR) and sorbitol dehydrogenase (SDH) make up the sorbitol pathway, which has been implicated in the pathogenesis of sugar cataracts. The levels of the two enzymes were determined with quantitative histochemical techniques in the epithelium, cortex, and nucleus of normal and diabetic rat lenses. The ratio of activity of AR to SDH was found to be nearly 50 to 1 in all substructures. This would strongly favor sorbitol accumulation and subsequent cataract formation.