The effect of long-term administered of methomyl on biochemical aspects, thyroid function and reproductive system was studied in male albino rats treated with methomyl soluble powder (90% SWP). Four groups of male mature albino rats (10 each) were administered 5, 10 and 20ppm of methomyl through drinking water for 90 days. The investigation results revealed that the methomyl significantly decrease .the (ALT) activity after 90 days at 10 and 20ppm of tested compound, while the activity of (AST) and total protein were no affect all over the period. The treatment of methomyl led to decrease the albumin level. The effect of tested material on thyroid function was assessed using measuring the thyroxin (T4) and tri-iodothyronine (T3) levels and histopatholological examination of thyroid gland. The effect of methomyl on male fertility was assessed using the mating method, spermatozoal, testosterone level and histopathological examination of the testis. The administration with methomyl caused decline in thyroxin (T4) and tri~ iodothyronine (T3) levels. Methomyl treatment caused reduction in sperm motility and concentration in male albino rats, in addition to homogenous eosinophelic material and hyperemic blood vessels inbetween the seminiferous tubules of testis. Moreover a reduction pregnant rats, litter size per pregnant females and litter size per females available were recorded.