Maleic hydrazide (MH) inhibits corn root elongation through an effect on cell division apparently without inhibiting cell enlargement. The decrease in the rate of elongation was apparent only after a considerable lag, over 14 hours, even with a concentration as high as 5 mM. MH (1 mM) did not inhibit His growth of roots from corn seeds given very large doses of Γ-irradiation or excised corn root segments including the elongation Zone or the cell enlargement induced by IAA in corn coleoptile sections. Many compounds including purines, pyrimidines, nucleosides. cysteine, pyridoxal, pyruvate. kinetin and CoCl 2 , many of which had previously been reported to alleviate MH inhibition in other tissues, were tested for their ability to prevent the inhibition of corn root elongation by MH, but none were effective. These data do not support the theory that MH acts by inhibiting the synthesis of or competing with some simple metabolite or hormone. Whatever its mechanism of action the failure of MH to inhibit cell enlargement in most systems indicates that it is fairly selective.