Albino rats were born and raised in 12 hr light: 12 hr dark regimes of illuminances varying from 3- to 800 lx. At 15 weeks of age, the animals were killed and determinations were made of the following: dark-adapted and steady-state rhodopsin levels; rod outer-segment length and photoreceptor-cell density; retinal topography of rhodopsin absorbance, and regeneration rate of visual pigment in vivo. It was found that there is a four-fold drop in the dark-adapted rhodopsin level of animals raised in 400-lx cyclic light compared with those raised in 3 lx. This difference can be accounted for by differences in rod outer-segment length and transverse absorbance of frozen retinal sections. Further, this change in rhodopsin content, coupled with variations in the visual pigment regeneration rate, allows the rat to control the amount of pigment in its retina at steady-state bleach. In this way, the rat can regulate the number of photons its retina catches each day. Animals raised in cyclic illuminances differing by more than two orders of magnitude catch very nearly equal number of photons (1.10 +/- 0.2 X 10(16) per eye) during the light period. A reduction in the number of photoreceptor cells also occurs with increasing illuminance, and these changes are more pronounced in the inferior region of the retina. This is not typical of the type of light-induced retinal damage caused by acute exposures.