Abstract-Methomyl (S-methyl-1-N [(methyl carbamoyl) oxy] thio acetimidate, is a carbamate broad spectrum insecticide. The Swiss albino mice were administered orally 1, 2, 3, and 4 mg/kg body wt methomyl for 30 days and effective dose 4 mg/kg for 5, 10, and 20 days to know the dose and durational effect on testes and biochemical contents. The mice were sacrificed on day 31 st or 24 hours after the terminal exposure. Testes, epididymis, vasa deferentia, seminal vesicles, prostate, coagulatory and Cowper's glands weight decreased significantly with 2, 3 and 4 mg/kg/day and in 4mg/kg /day for 10 and 20 days of methomyl treatment.Histologic studies of the testes revealed anti-spermatogenic and anti-steroidogenic effects of methomyl reflected by significant decrease in the number and diameter of spermatogonia, primary spematocytes, secondary spermatocytes, spermatids and Leydig cells in the mice treated with 2, 3 and 4 mg/kg/day and in 4 mg/kg /day for 10 and 20 days of methomyl treatment, except with 2 mg/kg/day methomyl treatment number of primary spematocytes, Leydig cells, diameter of spermatogonia, and in 4 mg/kg/day methomyl for 10 days treatment the number of spermatogonia, leydig cells and diameter of primary spermatocytes not changed significantly when compared with controls. Biochemical studies of testis and epididymis showed methomyl at doses of 2, 3 and 4 mg/kg/day and in 4 mg/kg /day for 10 and 20 days of methomyl treatment caused significant decrease in the levels of DNA, RNA, protein, glycogen and sialic acid whereas cholesterol increased significantly. However, treatment with 1, 2 mg / kg/ day and 4 mg / kg /day for 5 days methomyl caused no significant change in epididymal biochemical contents when compared with control. Study on enzyme activities in testis and epididymis revealed that the treatment with 3 and 4 mg/kg/day and 4 mg/kg /day for 20 days of methomyl caused significant decrease in the activities of succinic dehydrogenase (SDH), ions ATPase (Na + -K + ATPase, Ca ++ ATPase, Mg ++ ATPase) and acid phosphatase (ACP) were decreased significantly, except with 3 mg/kg/day methomyl treatment where Ca ++ ATPase activity in testes and epididymis not changed significantly. However, lactate dehydrogenase (LDH) and alkaline phosphatase (AKP) activities were increased significantly. The testis 3β hydroxysteroid dehydrogenase (3βHSD) decreased significantly with 3 and 4 mg/kg /day and in 4 mg/kg /day for 10 and 20 days of methomyl treatment when compared with controls. However, there was no significant change in testes and accessory reproductive organs, number and diameter of spermatogonia and Leydig cells and biochemical contents of testis and epididymis with 1 mg/kg/day and in 4 mg/kg /day for 5 days of methomyl treatment. These observed effects of methomyl on testes and accessory reproductive organs weight, testis histology, biochemical contents and enzyme activities in testis and epididymis may be due to toxic effect or hormonal imbalance in any of the stage in the hypothalamo-hypophysial-testicular ...