Location and in vitro determination of Na-K ATP'ase activity in the anterior structures of individual human lenses with senile cataract are reported, with special reference to anterior capsular/subcapsular opacity (ACSCO). Histochemically, ATP'ase reaction products were found exclusively in the epithelium. Even totally opaque lenses showed strong positive reactions. Biochemically, increasing ratios of Na+/K+ concentrations in the assay medium resulted in an increase in enzyme activity to a limited degree, whereafter the activity remained stable. We cannot decide whether the Na-K ATP'ase activity of the anterior lens structures is unchanged in relation to ACSCO as indicated by our figures. For there are methodological problems, although our analytical error, expressed as the variation coefficient for slaughterhouse pig lenses, seems to be one of the lowest so far reported in the literature on interindividual, non-pooled material.