1. The experiments described in this paper were performed with decapitated and non-decapitated Avena coleoptiles separated from the seed. They were immersed in water or in a growth-substance solution (usually 0.1 mg IAA/1). The geotropic response was studied with the aid of an apparatus similar to that described by Anker (1954). 2. Even in deseeded coleoptiles a regeneration of a physiological tip takes place, whose activity attains a constant though low rate after 2 to 3 hours and continues for at least 5 hours. In the presence of growth substance (0.1 mg IAA/1) in the external solution no regeneration of a physiological tip takes place. 3. In deseeded and decapitated coleoptiles the geotropic reaction as well as the geotropic perception are dependent on the presence of a growth substance, in this case lAA. 4. The uptake of growth substance is a rapid process, whereas the fixation of the growth substance to the protoplasm proceeds at a much slower rate. 30 to 45 minutes after its uptake in the cells all growth substance is bound. This means that, in the presence of growth substance, after about 30 minutes a steady state is established, i.e. an equilibrium between the rate of uptake, basipetal transport and fixation of growth substance, so that a certain constant amount of “free” growth substance remains available in the tissue. 5. In the absence of IAA no “preparatory processes” are induced by the unilateral influence of gravity in decapitated coleoptiles kept horizontally in water. For, if transferred into a growth-substance solution (0.1 mg IAA/1) and placed vertically, they do not curve and, if left horizontally after addition of IAA, they do not curve faster than coleoptiles which at the moment of growth-substance addition are given an antagonistic geotropic induction. 6. If an antagonistic geotropic induction is given to curved coleoptiles, the back curvature proceeds at a higher rate than the curvature in the original direction. It appeared that this was not due to the higher value of the angle between the tip and the vertical at the moment of rotation through 180°, but is caused by “growthtechnical” rather than by specific geotropic factors. 7. When horizontally placed coleoptiles (in growth substance) are either put vertically or rotated through 180° the development of the curvature according to the first geo-induction proceeds for ca. 30 minutes. This is apparently due to: (1) the IAA is fixed during the first induction and is no longer available and (2) the concentration of the free auxin is too low to cause a curvature in the opposite direction. The straightening of geotropically curved coleoptiles after an antagonistic induction starts at the base and proceeds acropetally. 8. Two short-lasting antagonistic exposures (10 or 20 minutes) do not neutralize each other. They induce two subsequent reactions. With non-decapitated coleoptiles the second reaction is stronger than the first and than that of decapitated coleoptiles in a growth-substance solution. This indicates a particular ability of the colcoptilar tip to regulate geotropism.