A possible involvement of two different systems in proton translocation was investigated by simultaneous measurement of transmembrane electron flow and proton secretion in a pH-stat combined with a redoxstat. The pH gradient between cytoplasm and apoplast is probably maintained by an H+ -pumping ATPase and by a second proton extrusion system, which seems to be linked to a redox chain with NAD(P)H as electron donor. Indole acetic acid inhibits both e− and H+ efflux, but only if the `electron draw' from the outside is not too high. The electron draw depends on the hexacyanoferrate level at the plasmalemma surface and on the Ca2+ concentration. The inhibiting effect of auxin on e− and H+ efflux in the presence of hexacyanoferrate can be only detected at low levels of bivalent cations and of the artificial electron acceptor. The inhibition of e− and H+ efflux by auxin requires high oxygen levels. The influence of auxin on both e− and H+ transfer disappears below 2 kilopascals O2, a level which does not influence respiration. Ethanol and fusicoccin do not increase the e− flux, probably because the electron transfer from the plasma membrane to HCF III is the limiting step. If electron transfer is reduced by IAA pretreatment, ethanol increases e− flux. Fusicoccin decreases e− and increases H+ efflux if the rates have been lowered previously by indole acetic acid pretreatment. This effect depends on high oxygen levels and is reversible by lowering oxygen pressure. Auxin and Ca2+ change e− flow and H+ ejection in a 1:1 ratio.