Mercury has been recognized as an industrial hazard that adversely affects male reproductive systems of humans and animals. However, less information is available concerning the underlying mechanism in the pathogenesis of male reproductive dysfunction. The present study investigated the possible involvement of oxidative stress to induce oxidative deterioration of testicular functions in adult rats. Wistar male rats (n=132) were continuously exposed to HgCl(2) at 0, 50 and 100 ppm during 90 days through oral administration in the drinking water. Mercury exposure for 90 days resulted in an increase in the absolute and relative wet weight of the testis and a decrease in the absolute and relative wet weight of the accessory sex glands, with respect to the matched control. Marked perturbation in testosterone serum level was also detected during the treatment for the treated groups. Cauda epididymal sperm count/motility decreased significantly in the mercury-treated group and qualitative examination of testicular sections revealed a fewer mature luminal spermatozoa in comparison to the control. When the mercury-treated males were mated with normal cyclic females, mercury exposure resulted in a decline of the reproductive performance of male rats. These effects were associated with a significant increase in mercury content of testes and blood in time-dependent and dose-dependent fashion, respectively. The HgCl(2) treatment was associated with oxidative stress. Evidence of induction of oxidative stress was obtained in terms of perturbations in antioxidant defense and a significant dose-dependent increase in the testicular lipid peroxidation as a consequence of pro-oxidant exposure. Taken together, the results suggest that an increase in free radical formation relative to loss of antioxidant defense system after mercury exposure may render testis more susceptible to oxidative damage leading to their functional inactivation.