The effects of oxidative stress in the adult male housefly were examined by the administration of 1 mM paraquat. Houseflies exhibit NADH and NADPH-diaphorase activity. Paraquat caused a significant decrease in life span, metabolic rate and the concentration of thiobarbituric acid-reactants. Concentrations of reduced glutathione and inorganic peroxides were increased by paraquat. Paraquat stimulated the activity of catalase but did not affect activities of superoxide dismutase and glutathione reductase. The levels of oxidized glutathione and the rate of fluorescent age pigment accumulation were unaffected by paraquat. Results indicate that paraquat toxicity does not result from lipid peroxidation.