Lens opacification of the Ihara hereditary cataract rat (ICR/1 rat) was followed up with slit lamp biomicroscopy and photography up to 1 year after birth. The development of lens opacities was first recognized in the anterior superficial cortex of the equatorial region between 30 and 44 days of age. It progressed to maturity around 93-107 days after birth. Cataract development was classified into six stages. The increase of lens weight indicated a continuous lens growth after birth which seemingly even continued after the occurrence of lens opacities. After around 60 days of age, the lens showed a continuous increase of water content, suggesting a participation of increased water content in lens opacification.