The different sensitivity of broad-leave weeds against the photosynthetic inhibitors phenmedipham and bentazon could be explained by the combined action of herbicide uptake and decomposition. In spite of fast uptake of phenmedipham by Beta vulgaris, this plant was not injured, because the herbicide was metabolized very rapidly. The transformation of phenmedipham by Galium aparine, Matricaria chamomilla, Centaurea cyanus, Stellaria media and Amaranthus retroflexus was reduced compared to Beta vulgaris. These plants survived, with the exception of Amaranthus, first of all because of a reduced uptake of the herbicide. The other weeds investigated (Galinsoga ciliala, G. parviflora, Sinapis alba, Capsella bursa-pastoris, Sinapis arvensis, Senecio vulgaris) were extremely sensitive against phenmedipham because they could not protect themselves neither by a reduced uptake nor an increased metabolism. The grass weeds Alopecurus myosuroides and Avena fatua are resistant against the herbicide because of the protected growing point and the missing basipetal translocation of the compound. Also the susceptibility of rice, soybeans, wheat and some weeds to bentazon is the result of penetration and metabolism of the herbicide in plants. Susceptible plants (Chrysanthemum segetum, Matricaria chamomilla, Xanlhium pensilvanicum) are nearly unable to detoxify bentazon. whereas in resistant plants (rice, soybeans, wheat) a substantial amount of absorbed bentazon was converted to different metabolites. Only Echinochloa crus-galli, which is more resistant, and Galium aparine, which is more susceptible, when they adsorbed the same quantity of bentazon, deviate from this general pattern.