The pre and post-natal development of wild type mouse lenses was studied by transmission electron microscopy, with special emphasis on denucleation of primary lens fibres. Denucleation of primary fibres is characterized by nuclear accumulation of small granules, most likely nucleosomes, which are condensed to osmiophilic bodies in the nucleus and in the cytoplasm. The osmiophilic bodies are laid down in apposition to the fibre membrane and are invaded by vesicles and granules, which probably contain proteolytic enzymes. Part of the breakdown products are extruded into the extracellular space, transported to the anterior and posterior poles where they might be finally digested or discarded from the lens. The morphology of the denucleation process of primary fibres is different from the gradual fading of nuclei in secondary fibres as described by Kuwabara and Imaizumi (1974: Invest. Ophthalmol. Vis. Sci. 13, 973-81).