The action of the herbicidal compound, 4,6-dinitro-o-cresol (DNOC), on rye roots was studied by analyzing its uptake from solutions and subsequent translocation by roots of intact plants and by isolated roots. The distribution of DNOC in the root cells was investigated by differential centrifugation of tissue homogenates. DNOC is readily accumulated and spreads throughout the plant by apoplastic movement. Redistribution hardly occurs. Next to uptake with the transpiration stream there is an intense passive absorption of mainly undissociated molecules. DNOC is partly adsorbed to the walls of xylem vessels and parenchymous cells, partly present in a soluble form. Although adsorption to protein sometimes occurs, DNOC is not metabolized. Soon after the accumulation, unaltered DNOC is released again into the outer solution by a mechanism sensitive to temperature and oxygen tension. The nature of this mechanism,its possible localization and role in detoxification and resumption of growth are discussed.