About 4 hours after they are cut from the seedling, corn (Zea mays L.) coleoptile segments mounted vertically show a strong increase in growth rate. This increase occurs in water or various buffers near pH 7 and is not accompanied by the accumulation of a growth promoter in the medium. The increase in growth rate is prevented by 1 mmp-fluorophenylalanine and is strongly inhibited by 0.1 mmp-chlorophenoxyisobutyric acid.The increased growth rate is accompanied by a 95% increase in the ability of tissue extracts to catalyze the conversion of (14)C-tryptophan to (14)C-indole-3-acetic acid and by a nearly 3-fold increase in indole-3-acetic acid oxidase activity. The increase in growth rate is also observed in segments from coleoptiles grown aseptically.The spontaneous increase in growth rate is completely but reversibly inhibited by 1 mum indole-3-acetic acid. Cytokinins have little effect on the spontaneous growth response, whereas gibberellic acid is observed to extend the latent period and reduce the magnitude of the response. It is tentatively concluded that the increase in endogenous growth rate may result from increased auxin production upon derepression of the auxin biosynthesis pathway after isolating the tissue from the normal supply of auxin from the tip.