Summary Determinations of Mg ATPase and Na/K ATPase activity in lens epithelium and in decapsulated lenses or lens fibers were made in lenses obtained from rats on normal and galactose diets at 0, 6, 15, and 23 days of the experimental period. Galactose feeding resulted in a marked lowering of Mg ATPase activity in the epithelium after 6 days on the diet. The level of Na/K ATPase activity was markedly decreased in the fibers after 15 days on the diet. The lowering of Na/K ATPase activities in the fibers may provide a possible link between the known decrease in the glutathione early in the cataractogenic process and the known destruction of fiber membranes that is associated with the appearance of mature cataracts.