Changes during the development and degeneration of the retina in 020/A mice, which are homozygous for the newly reported rds (retinal degeneration slow), gene were studied by histological and enzyme-histochemical methods with Balb/c mice carrying the normal allele as control. During normal development the total thickness of the retina grows from the time of birth till the age of 21 days and thereafter gradually diminishes, while the thicknesses of the component layers show a characteristic and differential change in course of their histogenesis. In the normal retina the perikarya of the cones are more frequent in the central than in the peripheral areas. The cone frequency in the central retina, but not in the periphery, increases with age and implies selective loss of rod cells in older animals. In the homozygous rds mice, the receptor layer remains rudimentary, but the other retinal layers show a normal trend of growth during the first 2 weeks after birth. Thereafter th morphological layers containing visual cell structures--the receptor, the outer nuclear, and the outer plexiform layers--begin to reduce. The loss of visual cells is readily marked by the reduction of the outer nuclear layer and is first evident at 2 weeks after birth. Degeneration is more rapid up to the age of 2-3 months, when the outer nuclear layer is reduced to half of its original thickness; thereafter degeneration progresses more slowly. The receptor and the outer plexiform layers are also simultaneously reduced. At 9 months, the peripheral parts of the retina, and at 12 months, the entire retina is completely lacking in visual cells. In the central retina of the mutant, rod and cone cell populations are equally affected up to the age of 6 months, as their relative frequency remains similar to the normal. In the peripheral retina, where cell loss is more pronounced, and in the central retina at 9 months an increase in relative frequency of cones is recorded and indicate increased susceptibility of the rods to later degenerative changes. The inner parts of the retina, including inner nuclear, inner plexiform, and ganglion cell layers, remain morphologically unaffected until irregular vascularization follows total loss of visual cells. The pigment epithelium is also affected at this late stage and appears depleted and patchy. In the normal retina, macrophages which are positively stained for the enzyme N-acetyl-beta-glucosaminidase appear in the inner layers with the growth of the retinal vasculature. In the mutant, increased frequency and stainability of the macrophages are discernible in the inner retina at 11 days. The macrophages migrate outwards and are observed in the outer nuclear layer and in the optic ventricle during the period of degeneration. These findings are compared with the observations in the other retinal degeneration mutants in rodents, and in retinitis pigmentosa in humans. The suitability of the rds mice as an animal model system for the human disease is emphasized.