Summary Analyses of rat lenses just before and just after the appearance of diabetic or galactose cataracts demonstrate that this event is accompanied by an abrupt disruption of the cell membranes of the lens. The concentrations of polyol, glucose and fructose in the lens approach those in the blood; and the extracellular space as determined with radioactive sorbitol is markedly increased. It is suggested that chemical changes in the cell membrane may hold the clue to the etiological mechanism of cataract formation.