Results from investigation of the gravitropic responses of the Avena coleoptile under simulated weightlessness are presented. The tests were conducted using the flight hardware, identified by NASA as the Gravitational Plant Physiology Facility (GPPF), designed to support the Spacelab experiment, GTHRES. Weightlessness or so-called microgravity-conditions were simulated by the use of a so-called somersault clinostat. The plants were gravitropically stimulated with different g-forces and stimulation durations on a centrifuge. Timelapse video pictures of the plants were taken during the last hour prior to the stimulation and in a period of 3 hours after. The gravitropic responses of the plants were analyzed with the aid of an image analyzing system. Dose-response curves for stimulation forces 0.2, 0.4, 0.6, 0.8, and 1.0 g with different stimulation times ranging from 2 to 250 min were achieved. The results show that the reciprocity between force and stimulation time may be valid for small stimulation doses (lower than 5 g min), but not for larger doses. Furthermore, the threshold values (i.e. the smallest stimulation the plants are able to detect) found by extrapolation of the dose response curves, are less than 30 g s. The results are discussed with respect to an accepted space experiment, GTHRES, where the corresponding experiments will be carried out in the same apparatus under true microgravity conditions.