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Earlier work from our laboratory showed that retinal rods of albino rats closely regulate the number of photons they absorb. This "photostasis number" is maintained at about 1016 per eye per day, regardless of the light intensity in the habitat or day length. We showed that the retina is plastic and that the regulation is achieved by modification of the optical density of rod cells. In the present study, we showed that the eyes of pigmented rats also maintain photostasis. Total rhodopsin in the dark-adapted eye is higher in pigmented than in albino rats. Replicating the lower levels seen in albino rats requires either pupillary dilation or use of light intensity that approaches that of outdoor lighting. Under those conditions, rod cell count is also significantly lower than that at light intensities normally used in animal colonies. Despite these variations in retinal light-absorbing properties, all animals absorbed the photostasis number of photons (1.4 × 1016 per eye per day), 13,000-15,000 photons per average rod per sec. This range of rates is well within that known to saturate mammalian rods and keep them unresponsive to further absorption. We therefore hypothesize that a function of photostasis is to render rods functionless during daylight hours, but this hypothesis requires in addition that most if not all rods absorb at these rates. To test this possibility, we measured the rate of photon absorptions across a wide range of retinal areas with an ocular transmission photometer (OTP) and found that most, if not all, rods are bsorbing at these rates. An interesting aspect of the OTP studies is that excised, pigmented rat eyes retain pupillary responses. These responses are circadian in nature: they anticipate light-period boundaries and shift only gradually after shifts in the photoperiod. [ABSTRACT FROM AUTHOR]