Age-related changes in the lenses of ddy strain mice, ranging from 4 to 24 months old, were studied histologically. The early change of aging lens was noted as a slight extension of fiber cells into the posterior capsule in the area behind the nuclear bow around 6 months of age. Those cell extensions formed villous projections reaching up to one half of the total capsule by 18 months of age. Another notable change was a decreasing cell population of the lens epithelium and the appearance of their nuclear inclusions. The latter contained rough endoplasmic reticulum, free ribosomes and filamentous matrix at the early stage and a mass of filamentous material at the later stage. This change was initiated by 12 months of age, and became profound with the increase of age. Moreover, the bow nuclei became markedly displaced posteriorly after 14 months of age. At about 18 months of age, when the pathological changes in both epithelial and bow regions became severe, the superficial cells in the posterior cortex showed swelling and atrophy, which soon led to the lens opacity due to accumulation of cell debris and fluid. These histopathological changes associated with aging appear to correspond to the incipient form of posterior subcapsular cataract which is detectable by clinical examinations.