The ability of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) to induce oxidative stress in various tissues of animals has been reported. The nature and mechanism of action of TCDD on the antioxidant system of sperm has not been studied. In the present study we have sought to investigate whether TCDD induces oxidative stress in the epididymal sperm of rats. Subchronic doses of TCDD (1, 10, and 100 ng/kg body weight per day) were administered orally to male Wistar strain rats for 45 days. After 24 h of the last treatment the rats were killed using diethyl ether. The epididymides were removed and cleared from the adhering tissues. Epididymal sperm were collected by cutting the epididymides into small pieces in Ham's F12 medium, and counted using a hemocytometer. The epididymal sperm counts in the TCDD-treated groups decreased in a dose-dependent manner from the control value of 8.2+/-0.14 x 10(8) to 5.31+/-0.15 x 10(8). Since a positive correlation (r=0.95; n=24) was observed between sperm count and DNA content of the epididymal sperm, DNA content was routinely used as an indicator of sperm count, and the results were expressed in terms of both protein and DNA. There was a significant decline in the activities of superoxide dismutase (40+/-2.17 to 27.1+/-0.76/mg protein and 32.41 to 18.07+/-0.76/mg DNA), catalase (2.49+/-0.13 to 2.03+/-0.05/mg protein and 2.01+/-0.05 to 1.35+/-0.05/mg DNA), glutathione reductase (71.2+/-3.87 to 48+/-1.79/mg protein and 57.58+/-1.52 to 31.94/mg DNA) and glutathione peroxidase (22.4+/-1.43 to 16.9+/-1.57/mg protein and 18.08+/-0.61 to 11.38+/-1.22/mg DNA) while there were increases in the levels of hydrogen peroxide (20.8+/-1.96 to 55.3+/-0.88/ mg protein and 16.18+/-1.88 to 36.87+/-0.88/ mg DNA) and lipid peroxidation (2.17+/-0.2 to 6.08/mg protein and 1.75+/-0.12 to 4.05+/-0.12/mg DNA) in the epididymal sperm. The results suggest that graded doses of TCDD elicit depletion of antioxidant defense system in sperm, indicating TCDD-induced oxidative stress in the epididymal sperm. In conclusion, the adverse effect on male reproduction in TCDD-treated rats may be due to the induction of oxidative stress in sperm.