In this study we examined the mechanism of action of Dieldrin on the reproductive function in adult male rats. Experimental data suggest that exposure to pesticide at 3 and 6mg/kg/day for 10 consecutive days, induced a dose-dependant decrease of the number and the mobility of epididymal spermatozoa. Tes-ticle histological analysis shows alteration of spermatogenesis with marked reduce of gametes production combined with reduction in the seminiferous tubule diameters in treated relative to control. Concentration of plasma testosterone dropped significantly in a dose-dependent manner after exposure to Dieldrin, while circulating levels of Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH) and weight of accessory sex glands were not affected significantly. Thus, it may be concluded that Dieldrin exposure can impair both testicle gametogenesis and steroidogenesis in adult rats.