Rod outer segment (ROS) disk-shedding patterns were determined in the albino rat eye. All of the animals had the same, low-intensity-light histories, but on the day that shedding patterns were to be measured the rats were divided into three groups: one which did not have any lights turned on at 0700 hr ('contained dark' group); one which had normal, 3-lx light turned on ('colony' group); one which had a brighter-than-normal light turned on ('80-lx' group). The 'continued dark' group displayed a broad profile of shedding over time which persisted because phagosomes disappeared so slowly. The 'colony' group showed the same initial rate of phagosome production as did the 'continued dark' group but a much accelerated rate of phagosome disappearance. The '80-lx' group showed no rate of phagosome production; rather there was a simple, rapid exponential disappearance of those phagosomes which existed just prior to light onset. The kinetics of these phenomena are described in a model, rate constants for which were obtained from our data and simulated on an analog computer. The analysis of the three shedding patterns indicates the existence of an intensity-dependent disappearance rate and suggests an inhibition of shedding and/or phagocytosis by intensity greater than that of the usual environment.