Abstract: Objective To study the effects of sodium arsenite on ROS level and TYR activity in G361 cells.Methods Human melanoma G361 cells were cultured with sodium arsenite at the concentration of 0.1,0.5,and 5.0 μmol/L for 2 and 6 hours,respectively.Cells were then measured for the levels of reactive oxygen species(ROS)and the actevities of tyrosinase(TYR).Results ROS levels of the 5 μmol/L for 2 hr(130.12±3.07%)and for 6 hr(139.55±11.02%)exposure group were significantly higher than that of control group.TYR activity was 99.31±0.81,99.85±1.34,99.52±1.43% for 2 hr guoup and 99.50±1.88,100.35±1.95,101.68±1.66% for 6 hr group,respeetively.There was no significant difference between arsenic exposed and control guoups in TYR activity.Conclusion ROS levels increase with the concentration of sodium arsenic exposure in a dose dependentmanner.