The transport and metabolism of naphthylacetic acid -I-14C in explants of Coleus was studied by means of the liquid scintillation counting technique in combination with thin layer chromatography. Microautoradiography was used to study the intracellular localization of the auxin. The gross radioactivity in the tissue is mainly fixed just below the donor block. In this part of the explant the radioactivity is present in the cytoplasm next to the cell wall. Although all the radioactivity in the receiver block represents the auxin itself, tissue extracts showed an intense metabolic turnover of the auxin. A study was made of the time-dependency of the production of several compounds and the ratio between the compounds in extracts of different parts of the explant. Finally, the chemical identity of the substances is discussed.