A relationship between the activity of NADH oxidase of the plasma membrane and the IAA-induced elongation growth of hypocotyl segments in etiolated soybean (Glycine max Merr.) seedlings was investigated. The plasma membrane NADH oxidase activity increased in parallel to IAA effect on elongation growth in hypocotyl segments. Actually, NADH oxidase activity was stimulated 3-fold by 1$mu$M IAA, and the elongation rate of segments was stimulated 10-fold by 10$mu$M IAA. The short-term elongation growth kinetics, however, showed that the IAA-induced elongation of hypocotyl segments was completely inhibited by plasma membrane redox inhibitors such as actinomycin D and adriamycin, at 80$mu$M and 50$mu$M respectively. In addition, 1 mM actinomycin D inhibited the IAA-stimulated NADH oxidase activity by about 80%. However, adriamycin had no effect on NADH oxidase activity of plasma membrane vesicles. Based on these results, the plasma membrane redox reactions seemed to be involved in IAA-induced elongation growth of hypocotyls, and the redox component responding to IAA was suggested to be NADH oxidase.