Effects of gamma-irradiation on the life span of the adult housefly (Musca domestica) were examined in order to elucidate the relationship between metabolic rate, irradiation and longevity. Adult houseflies of both sexes, exposed to 0, 20, 40 and 66 kR of gamma-radiation, were housed under conditions of relatively high or low physical activity. The mean longevity of high activity males exposed to 20 and 40 kR exhibited a significant increase, whereas the average life span of all female populations, low activity males, and high activity males exposed to 66 kR was significantly decreased following irradiation. Radiation exposure caused a reduction in the rate of oxygen consumption in both sexes as well as a decrease in the incidence of wing loss in the male flies. Analysis of the results suggests that radiation-induced life-lengthening in the housefly is a consequence of reduced metabolic activity.