An electron microscope study of lenses in 11--15 and 17 days old embryos of mice homozygous by dominant cataract-Fr (CatFr) gene has shown that ultrastructural changes in the nuclear envelope are the earliest expression of the mutant gene. The primary lens fibers of 12 days old embryos CatFr/CatFr, unlike those of the normal ones, are characterized by the decrease in the number of nuclear pores, evagination of the outer nuclear membrane, marked and unequal extension of perinuclear space which connects itself with the endoplasmic reticulum cisternae. In 14 days old embryos breaks in the outer nuclear membrane and evaginations in the inner one, fusion of nuclear membranes and breaks of the nuclear envelope are observed and resulted in the release of the nuclear contents with the nucleolus in the cytoplasm. Similar ultrastructural changes are characteristic also of the nuclei of secondary lens fibers at a comparable stage of differentiation. The destructive changes of nuclei are accompanied by the degeneration and autophagocytosis of cellular organelles and matrix.