Electron microscopic, freeze fracture and cholesterol cytochemical studies have revealed that the maturation of lens fibers is accompanied by considerable changes in the ultrastructure of their limiting membranes. These changes are in line with biochemical studies on the cholesterol content in the lens and support the physiological notion that upon maturation lens membranes are transformed from normal permeable to nonleaky membranes. In addition it is shown that early cortical changes involve disturbances of lens membranes. As a consequence it can be postulated that, in order to be significant, future studies on the early events leading to cataractous disturbances must be carried out using a combination of histochemical, immunohistochemical and regional biochemical approaches.