Abstract: In order to study the mechanism of different tolerance of various rice species or combinations against metsulfuron-methyl, the trial was carried out by the comparison between sensitivities of ALS to metsulfuron-methyl in vitro and in vivo. The results showed that the relationship between percentages of ALS specific activities and logarithms of metsulfuron-methyl concentrations in vitro was significantly negative linear correlation; ALS specific activities were inhibitted by metsulfuron-methyl in vivo, but not changed regularly according to the increasing of the concentration of metsulfuron-methyl. At the same metsulfuron-methyl concentrations, ALS specific activities in vivo were obviously higher than that of in vitro, the difference was well correspondent with the tolerance of rice plant. Therefore, it was induced that metsulfuron-methyl was strongly metabolized in rice and the difference of metabolic deactivation of metsulfuron-methyl was the base for mechanism of various rice species or combinations against metsulfuron-methyl.