The concentrations of sodium, potassium and chloride and the mannitol space have been studied in a group of carefully selected human senile cataractous lenses and in clear lenses from individuals of the same age group. Particular care was taken to select pure cortical and pure nuclear cataracts. In the course of senile cortical cataractogenesis, sodium accumulation within the lens represents a very early phenomenon which is associated with an increase of the space available to mannitol, a decrease of potassium and is followed, at later stages of the disease, by an uptake of chloride. Nuclear cataracts, on the contrary do not show appreciable modifications of sodium, potassium and chloride levels until the cortex is spared by opacification. The possible relevance of these findings to the mechanism of lens opacification is discussed.