Short-term applications of very high concentrations of l-naphthaleneacetic acid (NAA) to explants from flower stalks of tobacco (Nicotiana tabacum L. cv. Samsun) induced flower-bud regeneration to the same extent as longer or continuous incubation on lower concentrations. The maximum number of flower buds per explant after 15 d of culture was obtained not only by continuous culturing at 1 ~tmol.1-1 NAA but also by 12 h of culturing at 22 ~tmol-1-1 or 0.5 h at 220 ~tmol. 1-1, followed by incubation on medium without auxin for the remaining period. Continuous application of such high concentrations resulted in callus formation or caused the death of the explanted tissue. In all experiments in which auxin concentration and time of application were independently varied, the product of concentration and time determined the number of buds formed. Most, but not all, of the NAA taken up by the tissues was converted into conjugates. In explants which had received a dose which was optimal for regeneration, the internal concentration of free NAA remaining beyond the pulse period was between 1.7 and 6.2 Ixmol. 1-1. Suboptimal applications led to lower values, supraoptimal treatments to much higher internal concentrations. The physiological effect, which depends on the internal hormone concentration, thus manifested itself as dose-dependent with regard to applied hormone.