The enhancement by indoleacetic acid (IAA) of (36)Cl(-) uptake into Avena coleoptile sections was used to study the effects of a hormone on a membrane-controlled phenomenon. Compared to sections in phosphate buffer only, Cl(-) content of the cells increases 15 min after addition of IAA; the promotion is seen only with growth-active auxins and is saturated at 3 μM IAA. The percent enhancement by IAA is the same over a wide range of Cl(-) concentrations. The hormone effect is not observed at ice-bath temperature and is not correlated with growth or water movement into the cells. IAA does not influence the movement of Cl(-) in the section. While auxin must be present within the tissue in order to maintain the enhancement, there is no relationship between the total amount of auxin and the accelerated Cl(-) uptake that results. A polarity in the auxin effect is implied since only apical applications of IAA promote Cl(-) uptake.