The apical section (18 mm long) of a decapitated (1 mm removed) Avena coleoptile does not curve geotropically in water nor in a gibberellic acid solution. Gibberellic acid stimulates the geotropic curvature of similar sections carried out in indoleacetic and naphthylacetic acid solutions of a suboptimal concentration for the geotropism, but it inhibits the curvature if indoleacetic acid is supplied in a supraoptimal concentration. The inhibition requires much more gibberellic acid than is needed for the promotion. These results, together with data and suggestions from the literature, motivate an investigation into the possibility of a competitive synergism between gibberellic acid and auxins in the process of auxin transport. The regeneration of the physiological tip is not accelerated by gibberellic acid, but the geotropic curvature, made possible by the auxin production in the new tip, is increased.