As a component of diesel exhaust particles,3-methyl-4-nitrophenol(4-nitro-m-cresol,PNMC) is also a metabolite of the insecticide fenitrothion and imposes hazardous effects on human health.In the present study,the alleviative effect of a common antioxidant flavonoid quercetin on mouse germ cells intoxicated by PNMC was inves-tigated.Results showed that a single intraperitoneal injection of PNMC at 100 mg/kg induced severe testicular damage after one week.PNMC-treated mice showed a significant loss of germ cells(approximate 40% loss of round germ cells).PNMC caused an increase of hydroxyl radical and hydrogen peroxide production and lipid peroxidation,as well as a decrease in glutathione level,superoxide dismutase and glutathione peroxidase activities.Furthermore,treatment of PNMC increased expression of the pro-apoptotic protein Bax and decreased expression of the anti-apoptotic protein Bcl-XL in germ cells.In addition,testicular caspase-3 activity was significantly up-regulated and germ cell apoptosis was significantly increased in the PNMC-treated mice.In contrast,combined administration of quercetin at 75 mg/kg significantly attenuated PNMC-induced testicular toxicity.These results indicate that the antioxidant quercetin displays a remarkable protective effect on PNMC-induced oxidative damage in mouse testes and may represent an efficient supplement to attenuate reproductive toxicity by environmental toxicants to ensure healthy sperm production. As a component of diesel exhaust particles, 3-methyl-4-nitrophenol (4-nitro-m-cresol, PNMC) is also a metabolite of the insecticide fenitrothion and imposes hazardous effects on human health. In the present study, the alleviative effect of a common antioxidant flavonoid quercetin on mouse germ cells intoxicated by PNMC was inves-tigated. Results showed that a single intraperitoneal injection of PNMC at 100 mg / kg induced severe testicular damage after one week. PNMC-treated mice showed a significant loss of germ cells (approximate 40% loss of round germ cells). PNC caused an increase of hydroxyl radical and hydrogen peroxide production and lipid peroxidation, as well as a decrease in glutathione level, superoxide dismutase and glutathione peroxidase activities. Stillrther, treatment of PNMC increased expression of the pro-apoptotic protein Bax and decreased expression of the anti-apoptotic protein Bcl-XL in germ cells. In addition, testicular caspase-3 activity was significantly up-regulated and ger m cell apoptosis was significantly increased in the PNMC-treated mice. In contrast, combined administration of quercetin at 75 mg / kg significantly attenuated PNMC-induced testicular toxicity. These results indicate that the antioxidant quercetin displays a remarkable protective effect on PNMC-induced oxidative damage in mouse testes and may represent an efficient supplement to attenuate reproductive toxicity by environmental toxicants to ensure healthy sperm production.