The effect of light on the rate of visual cell loss in mice afflicted by the rds (retinal degeneration slow) gene was analyzed by comparing the changes in the thickness of the outer nuclear layer. Visual cell loss in the pigmented, homozygous mutant mice, maintained in cyclic light, is slightly slower than in the albino mutant mice. In the pigmented mutant mice, exposed to constant light, and in the albino mutant mice, reared in darkness, rate of cell loss is not significantly altered. In the albino animals exposed to constant light, the rate of cell loss is faster in the homozygous mutant than in the normal, and intermediate in the heterozygous mutant retina. The accelerated cell loss in the mutant retina progresses from the centre to the periphery, and affects the rods earlier than the cones. This resembles the photic lesion in the normal retina but is unlike the genetic lesion in the mutant retina which appears to progress from the periphery to the centre and affects both rods and cones. It is concluded that the visual cells in the retina of rds mutant mice are more vulnerable to photic damage than those in the retina of normal mice.