The lens wet weight differed from the age of 136 days: the male lenses were significantly heavier than the female lenses. The dry weight, in percent of the lens wet and dry weight, of the water-soluble proteins decreased and the dry weight of the water-insoluble proteins increased during ageing. At an age of 870 days, when the dry weight curves crossed each other, the lens contains equal amounts of water-soluble and water-insoluble proteins. At the end of the life expectancy, the relation between the water-soluble and water-insoluble moiety was 0.7 for the rat and 3.5 for the bovine lens. The dry weight percent of the water-soluble proteins was lower for the female lens, whereas that of the water-insoluble proteins was higher for the female lens. The male lens, on the contrary, showed a higher percentage of water-soluble and a lower percentage of water-insoluble proteins with increasing age. The ratio R of the dry weight of water-soluble to water-insoluble proteins was, from the age of 429 days, lower for the female lens. During ageing, the proportion of the lens-water was higher for the male lens.