The elongation rate of lupin hypocotyl and pea stem segments was measured every minute after the addition of various auxins and auxin precursors. After a latent period the growth-rate increased to a peak, fell to a minimum, and with most compounds increased to a second maximum. Compounds used include indol-3yl-acetic acid (IAA), indol-3yl-acetamide, naphth-2yl-oxyacetic acid, naphth- lyl-acetamide, and indol-3yl butyric acid. The extensibility of the cell walls of the lupin segments was measured with an Instron Universal Testing Instrument at intervals after the addition of IAA and it was shown that the lag period before the extensibility increased was longer than that for the growth-rate. Kinetic studies were made of the effect of Actinomycin D on IAA-induced growth. RNA synthesis during the first 20 and 40 min after IAA addition was also examined in segments exposed to labelled RNA precursors during these times. The results support the conclusion that RNA synthesis is not necessary for the initial action of auxin on elongation.