Abstract: Objective To investigate the effect and possible mechanisms of methyl parathion on testis cell cycle and cell apoptosis in male rats.Methods Twenty SD male rats(220～240g)were divided into four groups.The methyl parathion(MPT)was given orally to the rats for six weeks at the dose of 1.2,6,30 mg/kg/day,respectively.The testicular viscera coefficient was measu red and the activity of supe roxide(SOD),the contents of maleic dia ldehyde(M DA)were detected.The flowcy tometry was used to study the effects of MPT on testicle cells of male rats.Results After the exposure of 1.2,6,30 mg/kg M PT,the spermsurvival rates were 90.40%,82.99%,73.98% and the spermdeformity rates were 20.88%,23.91%,29.10%.Compared with the control group there were statistically significant changes(P<0.05,P<0.01).In 30 mg/kg group,the activity of SOD(185133 U/mg·pro)was lower that that of negative control group(344.61 U/mg·pro)and MDA con tent(1.90 nmol/mg·p ro)was higher than that of negative control group(0.71 nm ol/mg·pro,P<0.01).Compared with the control group,the proportion of cells in G0/G1 phase(73.43%,75.22%)and S-phase cells(12.0%,9.35%)were significantly lower in the middle and high dose group.In the high-dose group the apoptosisrate(32.77%)was higher than that of control group(22.17%,P<0.05).Conclusion The toxic effect of M PT on testicles of the rats is obvious,the oxidative stress may lead to changes in cell cycle and increase of apoptosis.