The effect of organophosphate insecticide dimethoate at three dosage levels (7, 15, and 28 mg/kg/day) on male reproduction in mice was studied. Dimethoate was given orally by gavage to male mice for 20 days before mating with untreated females. Signs of cholinergic effects were observed in the 15 and 28 mg/kg/day treated groups. Brain and skeletal muscle acetylcholinesterase activities were inhibited in both the middle and high dose groups. Dimethoate was associated with a decreased number of implantations and live fetuses, and an increased number of dead and early resorptions at 28 mg/kg/day treated group. The percent morphologically normal spermatozoa were unaffected in any of dose groups. However, sperm production and percent motile sperm were decreased in the 15 and 28 mg/kg/day treated groups compared to the control. Histological changes in testis were observed in the middle and high treated groups. The current study demonstrated the adverse effects of dimethoate on the reproductive performance of male mice and pregnancy outcomes following mating with untreated female mice at dose levels of 15 and 28 mg/kg/day. The No Observed Effect Level (NOEL) in the present study for reproductive performance was 7 mg/kg/day.