Mice homozygous for the rds (retinal degeneration slow) gene fail to develop receptor outer segments and show a slow loss of visual cells that starts from 14-21 postnatal days and results in complete absence at 1 year. In the heterozygous rds/+ mice the development of receptor outer segments is initially retarded. Although a distinct layer of outer segments of moderate length is formed, the disc structures remain disarrayed and form irregular whorls. Autoradiograms of rds/+ retinas show reduced incorporation of [3H]-leucine. Scleral movement of label, resulting from the addition of newly formed discs, is also retarded and appears irregular in comparison with the normal. Phagosomes, containing newly shed disc structures, within the retinal pigment epithelium of rds/+ mice are much larger than normal. Counts taken at different times of the dark- and light periods have shown an abnormally high turnover of phagosomes in the pigment epithelium of the rds/+ mice, with higher than normal peak frequency near the end of the light period, in contrast with the peak frequency in the normal pigment epithelium recorded around the beginning of the light period. Starting at 2 months, a very slow loss of visual cells, much slower than in the homozygous mutants, progresses throughout life. As a result, the outer nuclear layer at the age of 18 months or more is reduced to less than half. Prior to the reduction of the outer nuclear layer, the relative frequencies of the rod and cone perikarya in the rds/+ retina are similar to the normal values. With loss of visual cells, a small increase in the relative frequency of the cone perikarya is recorded in older rds/+ mice. This increase is more noticeable in the central than in the peripheral retina. The significance of the partial expression of the rds gene in the retina of the heterozygous mice in comparison with the changes observed in the homozygous retina is discussed. It is concluded that dose-dependent variation in phenotypic expression is an essential feature in the working of the rds gene.