Abstract BAS 662H, a 1:2.5 combination of the semicarbazone‐type auxin transport inhibitor diflufenzopyr and the auxin herbicide dicamba, is used as a post‐emergence herbicide in corn. The combination has been observed to provide more effective broadleaf weed control and improved tolerance in corn than typical rates of dicamba used alone. In order to analyze this phenomenon, the uptake, translocation, metabolism and action of both compounds, applied alone and in combination, were investigated in Amaranthus retroflexus L, Galium aparine L and corn ( Zea mays L). When plants at the third‐leaf stage were foliarly treated with diflufenzopyr and dicamba equivalent to field rates of 100 and 250 gha −1 , respectively, diflufenzopyr synergistically increased dicamba‐induced 1‐aminocyclopropane‐1‐carboxylic acid (ACC) synthase activity and ethylene formation in G aparine and even more in A retroflexus , followed by accumulations of (+)‐abscisic acid (ABA) in the shoot tissue within 20 h. This correlated with subsequent growth inhibition, hydrogen peroxide overproduction and progressive tissue damage. Diflufenzopyr also enhanced the activity of other auxin herbicides, such as quinclorac and picloram, and of the synthetic auxin, 1‐naphthaleneacetic acid. After foliar and root application of [ 14 C]diflufenzopyr, alone or as BAS 662H, considerably lower tissue concentrations and systemic translocation of radioactivity beyond treated plant parts were found in corn, compared to G aparine and particularly A retroflexus . Furthermore, diflufenzopyr decreased foliar uptake of [ 14 C]dicamba by c 50% selectively in corn, compared to the treatment alone. Metabolism of [ 14 C]diflufenzopyr was more rapid in corn than in the weed species. In combination, the two compounds had no mutual effect on their metabolic degradation. In BAS 662H, diflufenzopyr synergizes the herbicidal activity of dicamba in sensitive weed species. In corn this effect is prevented by a more rapid metabolism of diflufenzopyr, coupled with lower uptake and translocation. Selectivity of BAS 662H is additionally favoured by a higher crop tolerance to dicamba because of reduced foliar uptake of this herbicide in corn under the influence of diflufenzopyr. © 2002 Society of Chemical Industry