Exposure of human population to pesticides and industrial pollutants has considerably increased the risk of human health hazard. In the present study, therefore we have sought to investigate the toxic effects of Methyl Parathion on male reproductive system of rat. The tested dose was given orally to the rats for 30 days at the dose level of 30 mg/kg/day. Sex organs weight analysis, histochemical and histopathological changes and mating trials were the criteria used to evaluate the reproductive efficacy of the treated rats. The body weight of the animals did not show any significant change. However, Methyl Parathion caused significant decrease in the weight of testis, epididymis, seminal vesicle and ventral prostate with marked pathomorphological changes. Also, marked reduction in epididymial and testicular sperm counts in exposed males were noticed. Fertility test showed 80%, -ve fertility in treated animals. A significant reduction in the sialic acid contents of testis, epididymis, seminal vesicle, ventral prostate and testicular glycogen were noticed, while the protein and cholesterol content were raised significantly. From the above-mentioned findings, it has been concluded that exposure to Methyl Parathion has deleterious effects on male reproductive system of rat. Therefore, application of such insecticide should be limited to a designed program. (C) 2003 Elsevier B.V. All rights reserved.