The plasma membrane from Arabidopsis thaliana leaves (wild‐type and F mutant) has been purified by two‐phase partitioning and the H+‐transport activity was tested in vitro in the presence of different concentrations of IAA. While 1 µM of IAA had no effect, higher concentrations (10 µM and 100 µM) were inhibitory in 25 short days grown wild‐type plants. However, the activity was increased by about 16 % after 10 supplementary short days of growth (from 25 to 35 SD) in the represence of 10 µM of IAA. No significant sensitivity to the tested concentrations of auxin was observed during the development in short days (30, 38, 42 SD) or even the plants were induced by a 24 h of continuous light. The variability was not observed for a given developmental stage, from one experiment to another, but it was also observed after light induction of F mutant plants.