The plant hormone 3-indole acetic acid (IAA) is actively transported through coleoptile tissue (GOLDSMITH, 1969) and at the same time plays a regulatory role in at least two transport systems: proton secretion is promoted by hormone application (RAYLE, 1973) and auxin transport itself can also be modified by its substrate (HERTEL and FLORY, 1968). This regulatory function as well as the question of auxin control of cell elongation and tropisms will find a satisfactory explanation only by a model system which is made up of biochemical elements such as proteins, whose affinities towards auxins and other factors, conformational changes, enzymic activities, etc. are characterized and whose localization in the cell is understood; these elements and their interactions should then allow predictions of e.g. the early effects of the hormone, and the kinetic properties of the active transport system.