Albino rats and mice are sensitive to light and the recommended illumination of animal rooms (75-125 ft-candles) is known to cause retinal damage. When a room is illuminated by ceiling lights, animals in the cages of the top row and, to some extent, in the side columns of cage racks will be exposed to higher light intensity than those in the other cages of the rack. In 2-yr chemical carcinogenicity studies of the National Toxicology Program (previously the Carcinogenicity Bioassay Program of the National Cancer Institute), Fischer 344 rats were group-housed in hanging drawer-type clear polycarbonate cages. During the course of the chronic studies, a number of rats developed opacity of the eye. Ocular examination indicated chronic uveitis, deep interstitial keratitis, cataract formation leading to panophthalmitis, and in severe cases, phthisis bulbi. Histologic examination showed cataract and retinal degeneration. Incidences of these lesions were highest (greater than 55%) in the rats of the top rows and lowest in those of the bottom rows (less than 10%) of cages with no relation to chemical treatment, indicating an association with light intensity. The incidence of these eye lesions was markedly decreased (less than 15%) by decreasing the light intensity of the animal room to less than 50 ft-candles at 5 ft from the floor and rotating the cages in each column of a rack from top to bottom when cages or racks were changed.