The patterns of fluorescent age pigment (FAP) accumulation in response to gamma-irradiation were examined in adult male houseflies under conditions of relatively high and low levels of physical activity. Flies were exposed to 0, 20, 40, and 66 kR and their level of physical activity was altered by permitting or restricting flight activity. Under conditions of high activity the mean life span of flies exposed to 20 kR and 40 kR was greater than in the controls. The mean life span of high activity flies exposed to 66 kR and all groups of low activity irradiated flies was less than the control life span. The force of mortality as indicated by Gompertz slope was greater in high activity controls than in unirradiated flies maintained under low activity conditions. Gompertz slope was similar in corresponding high and low activity groups exposed to radiation. Radiation decreased the rate of FAP accumulation in high activity groups, but increased the rate of FAP accumulation under low activity conditions. The low activity control and low activity flies exposed to 20 kR had a lower FAP accumulation than the corresponding high activity groups at 14 days of age. At this age, low activity groups of flies exposed to 40 kR and 66 kR had concentrations of FAP that were similar to the corresponding high activity groups. These results indicate that radiation exposure decreased longevity by causing damage rather than by increasing the rate of aging.