The metabolism of the herbicide [uC]2-(2,4-dichlorophenoxy)-propionic acid (dichlorprop) was studied in excised seedlings of barley (Hordeum vulgare). It was rapidly taken up and metabolized by the plants yielding 5 metabolites. The metabolites, isolated by extraction with aqueous acetone, separated and purified by TLC, were identified by enzymatic, chemical, and spectrometric methods. Hydroxylation takes place at the 4-position of the aromatic ring, involving the NIH-shift of the chlorine to the 5-position and, to a minor extent, to the 3-position. The parent compound and also the 4-hydroxy derivatives undergo conjugation with mono-and diglu coses at the carboxyl and at the hydroxyl group, respectively. The time course of metabolism during 24, 48, and 72 h is presented, indicating interconversion reactions.