Elevated glucocorticoid levels during stressed conditions have been demonstrated to impair reproductive function in rats. In our previous study investigating the dose-related effects of corticosterone (CORT) on the fertilising capacity of epididymal sperm in surgically-manipulated rats, we found that 25 mg/kg/day of CORT given subcutaneously for seven consecutive days significantly decreased the number of implantation sites and increased intrauterine embryonic loss compared to controls. Based on these findings, the current study aims to elucidate the possible mechanisms of action of CORT-induced stress on impaired sperm fertility in rats. Results of the present study showed that compared to controls, 25 mg/kg/day of CORT given subcutaneously for 7 consecutive days significantly increased the level of plasma malondialdehyde (MDA) with corresponding attenuated levels of superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase (CAT) activities. Plasma adrenocorticotropin (ACTH) and testosterone levels were also found to be decreased in CORTtreated rats. These findings suggest that CORT-induced oxidative stress and exert an inhibitory effect at the hypothalamic-pituitary-gonadal (HPG) axis, as evidenced by increased lipid peroxidation, reduced enzymatic antioxidant activities, and decreased testosterone production. These subsequently result in decreased fertilising capacity of epididymal sperm leading to poor pregnancy outcomes.