An attempt has been made to simulate the light-induced oscillations of the membrane potential of Potamogeton lucens leaf cells in relation to the apoplastic pH changes. Previously it was demonstrated that the membrane potential of these cells can be described in terms of proton movements only. It is hypothesized that the membrane potential is determined by an electrogenic H(+)-ATPase with a variable H+/ATP stoichiometry. The stoichiometry shifts from a value of two in the dark to a value of one in the light. Moreover, this H+ pump shows the characteristics of either a pump or a passive H+ conductance: the mode of operation of the H+ translocator is considered to be regulated by the external pH. The pump conductance is assumed to be dominant at low or neutral pH, while the passive H+ conductance becomes more significant at alkaline pH. The pH dependence of the transport characteristic is expressed by protonation reactions in the plasma membrane. The proposed model can account for most features of the light-induced oscillations but not for the absolute level of the membrane potential.