Biochemical studies have demonstrated the presence of ouabain-sensitive Na-K-ATPase in lens epithelium. The development of certain experimental cataracts has been associated with the decrease in the level of activity of this enzyme. Cytochemical investigations at the ultrastructural level to localize this important enzyme in the ocular lens are needed to exhibit the site of Na-K-ATPase action. In this investigation cytochemical methods originally described by Ernst for the localization of Na-k-atpase through K-NPPase reaction have been used for normal and cataractous ocular lens. The reaction product was observed to be present on the lateral and apical portion of the epithelial cell membranes. Mooreover, the membranes of the cortical fibers in the anterior polar and equatorial region of the lens also exhibited reaction product of Na-k-atpase. The presence of ouabain reduced the reaction product of Na-K-ATPase; however, continuous exposure to ouabain during preincubation fixation, and incubation demonstrated greater reduction in enzyme action in the lens. The Na-K-ATPase activity in the lenses of animals fed galactose decreased with increase in time on galactose diet. In this study, using ultrastructural cytochemistry, we have demonstrated gradual decrease in Na-K-ATPase activity in the lenses of galactose-fed animals.