The response of Avena coleoptile sections to high concentrations of auxin has been determined in the absence of all additives except sucrose. In most experiments the growth-time curves with 75 p.p.m. IAA showed two linear phases. In the first phase, which lasted for only 2–4 hours, extension was as rapid with 75 p.p.m. IAA as with 5 p.p.m. IAA. This rapid initial expansion phase was then succeeded by a second phase which persisted for at least 20 hours. During this second linear phase the growth-rate with 75 p.p.m. IAA was lower than with an auxin concentration of 5 p.p.m. In some experiments the first phase was absent and only the second phase was present. The response of sections to high concentrations of auxin was not influenced by the presence of buffers or absorbable cations. Omission of sucrose or the presence of moderate amounts of ethanol caused the resulting growth curves to be non-linear. The rate of uptake of auxin into the tissues was dependent on the auxin concentration and was constant for at least 24 hours.