The skin penetration rate of methidathion in vitro and pharmacokinetics of methidathion in vivo were studied with male Sprague-Dawley rats by dermal treatment. The in vitro skin penetration rates for Sprague-Dawley rats of methidathion technical (50 mg, 100 ${mu}ell$) and emulsifable concentrate (EC,40mg, 100${mu}ell$) were determined as 18.4 $mu extrm{g}$/c $m^2$/h (RSD : 6.5) and 18.5 $mu extrm{g}$/c $m^2$/h (RSD : 3.2), respectively. Dose-related systemic exposure (AUC) was observed in rats after dermal treatment. The corresponding AUC, $T_{max}$, $C_{max}$, and $T_{1}$2/ of methidathion in plasma were 1.5$mu extrm{g}$.hr/ml, 6 h, 0.10 $mu extrm{g}$/ml, and 16 h, for 116mg/kg doses, 3.2 $mu extrm{g}$. hr/ml, 8 h, 0.12 $mu extrm{g}$/ml, and 23 h, for 232 mg/kg doses and 10 $mu extrm{g}$. hr/ml, 12 h, 0.32 $mu extrm{g}$/ml, and 20 h, for 1,160 mg/kg doses respectively. The urinary excretion of methidathion, estimated wing an equation derived from the in vitro skin penetration study was 0.24~0.35% of the absorbed dose. The concentration of methidathion in kidney was higher than that in liver. Dose-dependent absorption and excretion of methidathion without saturation was observed under in vivo experimental condition.n.n.