Effects of pyrazosulfuron-ethyl [ethyl 5-[[3-(4, 6-dimethoxypyrimidin-2-yl) ureido] sulfonyl]-1H-1-methylpyrazole-4-carboxylate] on the growth and activity of acetolactate synthase in rice (Oryza sativa) and C. serotinus plant were investigated. The growth of root and shoot was suppressed more remarkably in C. serotinus than in rice. In vitro, the enzyme activity from each organ of both plants was highly and similarly sensitive to pyrazosulfuron-ethyl; however, in vivo the activity was reduced more remarkably in C. serotinus organs after treatment with pyrazosulfuron-ethyl. Only in rice shoot was the reduction of enzyme activity in vivo remarkably recovered as a function of time after treatment. It is presumed that the primary site of action of pyrazosulfuron-ethyl is acetolactate synthase, and that its selectivity depends on the difference in degree of inhibition on acetolactate synthase activity in vivo as a function of time.