The character of the enzymic conversion of IAA in 48 hours old roots of Pisum sativum var. ‘Vlijmse Gele Krombek’ has been investigated in enzyme preparations as well as in the root tip tissue. From experiments with enzyme preparations it appeared that IAA is not converted by a peroxidase but by an oxidase. This oxidase possibly is a phenoloxidase. The latter has the capacity to convert DCP and p-cumaric acid in vitro. This conversion enhances the degradation of IAA in the solution. IAA is not converted if this solution does not contain a naturally occurring or added monophenol. The conversion of IAA therefore is linked with that of a monophenol as a co-factor. Caffeic acid (a diphenolic compound) inhibits the degradation of IAA in a solution. Also in the tissue of the root tips, the conversion of IAA is enhanced by the addition of DCP or of p-cumaric acid and inhibited or even checked by addition of caffeic acid. DCP itself is converted in the root. Apparently the same IAA converting system is active as well in the living tissue as in enzyme preparations in vitro. No influence of externally supplied IAA could be found on the rate of the IAA conversion in the root tips. In geotropically exposed, i.e. horizontally placed, roots the activity of the IAA converting system is lowered. This points towards a physiological meaning of this system. The effect of DCP, p-cumaric acid and caffeic acid has been investigated in relation to their effect upon the IAA converting system on the geotropic reaction and on the straight growth. In these experiments the roots had to grow in water or in solution of the mentioned compounds. Therefore the geotropic reaction, i.e. the course of the curvature and the growth rate at the upper and at the lower side of the horizontal root, was followed in different media; moist air, in water and in solutions of the mentioned compounds. The geotropic reaction shows alternating positive and negative phases, which are more pronounced in submersed roots than in roots growing in moist air. The effect of DCP, p-cumaric acid and caffeic acid on the geotropic reaction and on the straight growth suggest that IAA actually is the regulating factor of both processes in roots. No indication has been found that IAA is present in the root in a supraoptimal concentration. The observed phenomena are therefore incompatible with the Cholodny-Went theory in its classic form. If DCP is added at different stages of the geotropic reaction, it suppresses the next positive or negative phase of this reaction. Caffeic acid on the other hand promotes the origination of a difference in growth rate between the upper and the lower side of the horizontal root and prolongs the duration of such a difference. IAA added to the medium decreases the difference in growth rate between the upper and the lower side. The results endorse the conception that IAA regulates the geotropic reaction and that the IAA concentration in its turn is governed by the activity of the IAA converting system. If IAA is actually unequally distributed in the horizontal root, this unequal distribution has not necessarily to be ascribed to some lateral transport, but more likely to a different activity of the IAA converting system at the upper and at the lower side of the root tip. Finally the changes of several cell properties—suction force, osmotic value, wall pressure, plastic and elastic extensibility of the cell walls—have been determined in geotropically exposed roots. The influence of DCP, caffeic acid and IAA on these properties has been studied. The suction force, the osmotic value and the elastic and plastic extensibility are lowered after placing the roots horizontal (measured after 20 to 30 minutes). Although these magnitudes are changed by the addition of DCP, caffeic acid and IAA in vertically growing roots, similar changes were not observed in horizontal roots, whereas these substances do affect the geotropic reaction. In experiments of long duration the elastic and plastic extensibility of the cell walls and the water uptake show changes in water and in solutions of caffeic acid, the direction of which is opposite to that of the geotropic reaction. In solutions of DCP and IAA, however, no correlation has been found between these changes and the geotropic reaction.