A quantitative analysis of cell division and cell elongation was carried out during lens morphogenesis in the rat. At 13 days of development elongating cells in the posterior part of the lens vesicle (presumptive fibre cells) have a lower mitotic activity than cells in the anterior vesicle. By 14 days these elongating cells do not divide. Thus at 14 days of development the lens can be separated into two compartments; a proliferation compartment in the anterior lens and an elongation compartment in the posterior lens. The three main groups of lens-specific proteins, alpha-, beta- and gamma-crystallins, were localized by immunofluorescence. alpha-crystallin is the first crystallin to be detected and is localized in some lens pit cells at 12 days of development. By 14 days all lens cells contain alpha-crystallin. beta- and gamma-crystallins are detected later at 12 1/2 days and are localized in some cells situated primarily in the posterior part of the lens vesicle. At later stages of development these crystallins are restricted to cells of the elongation compartment, i.e. presumptive fibre and fibre cells. Possible mechanisms that govern the temporal and spatial distribution of crystallins are discussed.