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Growth inhibition by applied indoleacetic acid (IAA), naphthaleneacetic acid (NAA), or 2,4-dichlorophenoxyacetic acid (2,4-D) was studied using change in fresh weight of pea root segments as the criterion of growth. Auxin metabolism of these tissues was investigated with 14C-labeled auxins applied under conditions similar to those used in the growth studies.Growth inhibition by applied auxins is independent of the rate of auxin uptake, accumulation of auxin or auxin metabolites in the tissues, or the subsequent loss of accumulated auxin from the tissues. It is also independent of the metabolic processes leading either to auxin conjugation with aspartic acid or to decarboxylation. All three auxins inhibit growth to a similar degree, which depends only on the concentration of auxin applied and the pH of the solution. Inhibition persists undiminished as long as the auxin is applied. It is suggested that growth inhibition by applied auxin occurs at a site external to the cytoplasm, i.e. the cell wall or the cytoplasmic membrane.Growth inhibition of tissues after auxin treatment has ceased is not due to the auxin remaining in the tissues but rather to the auxin released from the tissues to the solution to which they have been transferred. Untreated tissues incubated in the same transfer solution with treated tissues are equally inhibited. The persistence of growth inhibition after treatment depends upon the ability of the tissues to convert accumulated auxins to physiologically inactive metabolites. Conjugation with aspartic acid accounts for the inactivation of all the accumulated NAA metabolized and the major part of the IAA. IAA decarboxylation under these conditions plays a lesser role. Growth recovery following treatment with IAA or NAA occurs as these auxins are metabolized. 2,4-D is not metabolized to any appreciable extent during these studies, and tissues remain inhibited to a degree consistent with the concentration of 2,4-D in the transfer solution.