A systematic study was made of the kinetics of geotropic response of coleoptiles of Avena sativa L., cv. Victory.Over a reasonably large range, response could be approximated as a linear function of stimulus duration for coleoptiles stimulated by horizontal placement. The threshold for induction of subsequently visible curvature was about 20 s, but evidence is presented that inductive reactions begin as soon as the coleoptile is set on its side. Corroboratively, intermittently administered stimuli as brief as 0.5 s were summed without loss.Plants were stimulated at selected angles between 0° and 90° from the vertical position of equilibrium for an arbitrarily selected time, and response was linearly dependent on the perpendicular component of acceleration. This dependence supports the hypothesis that geotropism is a response to the differential exertion of pressure. Data for supplementary angles are also consistent with the pressure hypothesis.