The general objective of this study was to identify biochemical correlates of longevity in the housefly by comparing two strains of flies that have different longevities. The average and the maximum life spans of the longer-lived "Cambridge" strain flies were 46% and 23%, respectively, greater than the shorter-lived "Thuron" strain flies. The hypothesis that longer-lived organisms have relatively more efficient mechanisms to minimize oxidative stress and maintain a relatively more reduced redox potential was tested. All measurements were made on 8-day-old male flies maintained under identical conditions. Flies of the longer-lived strain had a lower metabolic rate and contained lesser amounts of H2O2 and thiobarbituric acid-reactants than the flies of the shorter-lived strain. Reduced glutathione concentration and activities of catalase, glutathione reductase and thioltransferase were higher in the longer-lived strain indicating that longer-lived flies manifest lower levels of oxidative stress and greater ability to maintain a relatively more reducing environment than the shorter-lived flies. Superoxide dismutase (SOD) activity was similar in the two strains, but the SOD/metabolic rate ratio was higher in the longer-lived strain. Total activity of glutathione S-transferases was comparable in the two strains suggesting that differences in detoxification ability are not correlated with longevity. Only S-glutamylcysteine synthetase activity was greater in the shorter-lived strain suggesting that variation in longevity is not due to reduction in the ability to synthesize GSH. Overall, the results support the view that parameters associated with oxidative stress play a role in the aging process of the houseflies.