A method was developed to measure every minute the growth of a single segment excised from the elongating region of a plant. The method was used to determine the short term kinetics of growth in response to auxin addition. The method is not dependant on the use of hollow coleoptile tissue and the results are plotted as a growth rate against time. The technique has a resolution an order of magnitude higher than those in current use. The results show that there is a latent phase before auxin-induced increase in elongation rate occurs. After this latent phase, there is a rapid rise in rate to a maximum followed by a decrease and then usually a rise to a second maximum. Three hypotheses for explaining the growth rate curve are considered. It was found that neither RNA nor protein synthesis were required for the initial action of auxin but that protein synthesis became necessary within a few minutes after auxin addition. The apparent half-life of the protein whose synthesis is stimulated by auxin is about 12 min. This short half life suggests that, after the synthesis of the protein, there is a limited time during which it can act with auxin to increase elongation. A model which incorporated these results has been proposed and its relationship to the three hypothesis for explaining the growth rate curve is discussed.