A study was made of the effect of indole-3-acetic acid (I.A.A.) on growth and water intake of isolated Avena coleoptile sections in distilled water. Both processes were greatly promoted by I.A.A. Attention was focussed on the mechanism of the action of I.A.A. I.A.A. (1 mg/1) showed a tendency to decrease the permeability to heavy water, while a low pH increased this magnitude. The effect of I.A.A. and pH on growth and water intake could not be explained by their effect on the water permeability. The sections always maintained a positive suction force, even after prolonged immersion in I.A.A. solutions or in distilled water. This suction force, which is found after prolonged immersion, originated from the active growth of the sections and was significantly increased after short times of treatment with I.A.A. The osmotic value of the cell contents was not increased by I.A.A., but it decreased in proportion to the increase in length of the sections. I.A.A. did not stimulate the formation of osmotically active substances. The elastic extension of the sections was increased by I.A.A. This indicated a change of the cell wall. It was demonstrated that the greatest increase in elastic extension occurred during the first two hours after the addition of I.A.A. No direct relation could be established between the elastic extension and the growth rate. A further analysis showed that preceding plasmolysis decreased the effect of I.A.A. on the extension. Sucrose and glucose increased the effect of I.A.A., which increase might be partly due to a possible increase in the osmotic value by sugars. Dinitrophenol inhibited the action of I.A.A. on extension and on growth. These results indicated that I.A.A. changed the cell wall through intermediary of the active metabolism of the protoplasm. It is suggested that elongation is an active growth in area of the cell wall by intussusception. I.A.A. is assumed to promote the biosynthesis of wall substances by a physico-chemical stimulation of one or more enzyme systems. This hypothesis accounts both for the high energy consumption and for the change in wall properties after the addition of growth substances to coleoptile sections.