Na/K ATPase activity has been determined in normal and cataractous human lenses using labeled ATP as a substrate. The enzyme is distributed throughout the lens with approximately 1/3 of the total activity in the capsule-epithelium and 1/2 of the activity in the cortex. Furthermore, the activity of Na/K ATPase decreases with increasing age and this decrease occurs primarily in the inner nuclear region. In severe cataractous lenses, a marked decrease in the Na/K ATPase activity has been demonstrated in all parts of the lens, in contrast to immature cataracts, where the decrease in the enzyme activity occurs primarily in the cortical and nuclear regions. A report of this investigation was presented at the 1931 ARVD meeting in Sarasota, Florida.