Albino rats were born and raised through 12 weeks of age in 12L:12D regimes of 5, 300- or 800-lx illuminance. Upon killing, the animals' retinas were examined for the following: (1) rhodopsin of whole retina and isolated rod outer-segment membrane; (2) retinal morphology, including outer segment length and outer nuclear layer area; and (3) outer-segment membrane lipid biochemistry. The three groups of animals show significant differences with respect to one another for nearly every parameter measured. Rod outer-segment membranes of rats raised in dim cyclic light (5 lx) have high rhodopsin packing densities, high levels of polyunsaturated fatty acids, and low cholesterol levels in comparison with animals raised in brighter illuminances (300- or 800 lx). The mole ratio of phospholipid to rhodopsin in the outer-segment membrane of rats raised in 5-lx cyclic light is only 43% of that of rats raised in 800-lx cyclic light. The difference between these two groups of animals for docosahexaenoic acid is greater than three times, with dim light-reared animals showing higher levels. These rats (5 lx-reared) have less cholesterol in their photoreceptor outer segments, 6.6 mol% compared with 19.7 mol% for animals from the 800-lx regime. In all cases, rats from the intermediate rearing illuminance (300 lx) exhibit intermediate membrane composition values. It is likely that these differences in membrane composition illustrate a profound effect of light history on photoreceptor function.