Lenses of Emory mice, ranging in age from 7 days to 15 months, were studied by light and electron microscopy. The earliest change was acellularity in the lens epithelium, which was found at postnatal day 14. This acellularity was relatively mild but constantly occurred throughout the experimental course. The early cortical fiber change was swelling in the anterior region of the lens at 1 month of age. At 2 months of age some cell extensions towards the capsule appeared at the basal surface of the posterior cortical fibers near the equatorial region, and their height and number had greatly increased by 4 months of age. Following this change, there was an enlargement of nuclei in the bow area, swelling of cortical fibers, and separation of the posterior suture line. These changes became severe by 6 months of age. When the separation of the posterior suture and disturbance of the posterior cortical fibers became markedly severe, the cataract reached a mature stage, showing the complete opacity of the cortex. This animal model appears to indicate that the viability and/or behavior of the epithelial cells is important for maintaining the transparency of the lens throughout the lifespan.