Herbicides show many kinds of mode of action and are metabolized in many ways. These biochemical patterns are closely related to the efficacy or mammalian toxicity and also to the selectivity of environmental persistance of herbicides, respectively. In this short review, oxygenase will be discussed as to the metabolisms of monuron, EPTC and bensulfuron methyl, and active oxygens will be reviewed from the standpoint of the mode of action of paraquat and diphenylether herbicides.