Thin-layer isoelectric focusing of the water-soluble crystallins shows with increasing age of the animals an increase of α-crystallins of high and low molecular weight and of the total β-crystallins content. The β-crystallin components of high molecular weight also increased during ageing, but the β-crystallins of lower molecular weight decreased considerably. The total γ-crystallin content, as well as certain γ-crystallin components also decreased considerably with increasing age, or vanished completely. From 429 to 1,211 days of age the male γ-crystallins as percent of the water-soluble moiety, were higher than the female γ-crystallins. As a consequence of ascending age, the content of water-insoluble proteins of the rat lens increased gradually and continuously, e.g. by a process of insolubilization of soluble β- and γ-crystallins.