A model of retinal degeneration has been developed in mice which is induced by the DNA alkylating agent methylnitrosourea. Pregnant mice were injected with various doses of this potent teratogen on day 16 of gestation, a time of differentiation of numerous cell types of the mouse retina. Histological examination of retinas from offspring exposed to 20-, 10- and 5 mg kg-1 doses demonstrated retinal rosettes, a pathology similar to retinal dysplasia. The 1 mg kg-1 dosage did not produce rosettes; in fact, retinas appeared morphologically normal early in life. Control and treated animals were studied at specific age intervals: 2-, 4-, 6-, 8-, 12-, 16-, 20-, 36-, 52 weeks. Measurements of the overall retinal width and five retinal layers were made to quantify the degeneration. Results indicate a thinning of the retina begins at 4 weeks and worsens with age. These results are discussed with respect to the potential of low-level exposure to environmental toxins as a possible cause of retinal degeneration.