Extracts of etiolated pea seedlings oxidize indole-3-acetic acid, a plant auxin, to 3-hydroxymethyloxindole. At physiological pH this compound is dehydrated to 3-methyleneoxindole, a highly reactive sulfhydryl reagent and inhibitor of cell growth. 3-Methlyeneoxindole is in turn detoxified by an enzymatic, triphosphopyridine nucleotide-linked reduction to 3-methyloxindole, a nontoxic compound. These enzymatic conversions may be responsible for some of the responses to auxin, particularly sensitivity to its inhibitory effects on growth.