The role of glutathione as an antioxidant was examined in the adult housefly by the administration of diamide, an oxidant of glutathione. Although diamide effectively oxidized glutathione, it also stimulated the synthesis of glutathione. Diamide induced a decrease in the rate of oxygen consumption and in the activity of superoxide dismutase. Diamide increased catalase activity and the concentration of inorganic peroxides at early ages. The results suggest that oxidation of glutathione induces counterbalancing adaptive changes in the housefly.