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Quantitative analysis of indolyl-3-acetic acid (IAA) in different parts of maize root tips (Zea mays L. cv. Kelvedon 33) were performed by selective ion current monitoring detection (or mass fragmentography). It was found that the cap contained 356.6±16.2 μg of IAA kg(-1) fresh weight; the apex had only half this concentration of IAA. Calculations indicated that the 10-mm apical segments of the root contained 109.6 μg kg(-1) fresh weight. The possibility that the growth regulator formed in the cap and inhibiting the elongation of the extension zone of the root is IAA is discussed.