Young and senescent albino A/J mice, pigmented C57BL/6J pure inbred mice, and their hybrid F1S were tested under low or high illumination intensities to observe differences in self-selected wheel running, bar pressing, and durations of light and dark over time. The animals (N = 120) were always allowed ad lib access to food, water, running wheel, and bar-press levers. During the pre- and postexperimental phases, the mice were kept under a standard 12:12 hr light/dark cycle; during the experimental phase, however, they were allowed to select their own light and dark schedules by pressing on either of two accessible bars, one light contingent and the other dark contingent. Measures of general running and bar-pressing activities, motivational aspects of illumination change and intensity preferences, time-series analyses of periodicities, power ratios, and significant other multiples were obtained from the subjects during a total of three experimental phases. Age differences were found for most of the measures studied and in general showed declines in activity levels, inccreases in motivation to change illumination conditions, lengthening of activity cycles (slower periods), and decreases in the strengths of the oscillations underlying these behaviors as well as an increase in the number of other periodic components in old mice relative to young. Genetic group and illumination-intensity differences were also found, and the results are discussed in light of theories concerning illumination preference and stimulus change, earlier work involving voluntary light selection behavior, and aging studies.