SummaryPathways of chlortoluron metabolism were compared in excised leaves of four tolerant species, namely wheat (Triticum aestivumvar Clement),Bromus sterilis, Galium aparineandVeronica persica.The herbicide was principally detoxified by hydroxylation of the ring methyl in wheat and byN‐dealkylation inVeronica persica.Both pathways were involved inBromus sterilisandGalium aparine.Kinetic study of the degradation showed that capacity to form non‐toxic conjugates could, at least partially, explain the tolerance of these species to chlortoluron. In plants treated with 1‐aminobenzotriazole, a cytochrome P‐450 enzyme inactivator,N‐dealkylation of chlortoluron was little or not affected, but ring methyl hydroxylation was strongly inhibited. This suggests that at least two distinct enzymatic systems could participate in this metabolism. Moreover, cytochrome P‐450 enzymes could be involved in the ring methyl hydroxylati