Summary. Translocation of 3‐amino‐1,2,4‐triazole‐5‐14C (14C‐aminotriazole) was compared to that of 14C‐assimilates in couch grass (Agropyron repens (L.) Beauv.) at three different growth stages. Assimilates of 14CO2 were translocated from the treated shoot to other shoots and rhizomes at the 2–3‐leaf and 3–4‐leaf stages of development. Much less labelled material was translocated into untreated shoots at the 5‐leaf stage. More 14C‐assimilates were translocated to the roots than to untreated shoots at all developmental stages. The translocation patterns of 14C‐aminotriazole and 14C‐assimilates were similar. Two metabolites, A and B, were formed from 14C‐aminotriazole, which chromatographed identically to previously described metabolites in Cirsium arvense (L.) Scop. A was further metabolized into B. Labelled aminotriazole and its two metabolites were translocated throughout the plants. Metabolite A was phytotoxic when concentrated and re‐applied to couch grass, but its properties were not those of Unknown II from Cirsium arvense. They were the same as those of Unknown III. Whether or not metabolite A and Unknown III are identical was not established. Migration des substances assimilables marquées au 14 C, du 3 amino‐1,2,4‐triazole et de ses métabolites chez Agropyron repens