An attempt has been made to quantify the light-induced H+ efflux and influx observed in polar leaves of Potamogeton lucens. These proton fluxes are spatially separated. The H+ efflux, mediated by a plasmalemma bound H+-ATPase, occurs across the plasma membrane at the morphological lower epidermis and is accompanied by an H+ influx (or OH- efflux) at the upper side of the leaf. As a result, these leaves exhibit a remarkable pH-polarity in the light. The pH near the lower epidermis may drop to a value as low as 3.5, while a pH of about 10.5 can be observed at the upper epidermis. Obviously this phenomenon requires the co-ordination of transport processes in the different cell layers of the leaf tissue. These observations led to quantitative studies of the relation between the H+ fluxes at either plasmalemma. These fluxes were calculated from the pH values recorded at two distances from the leaf surface. Although the H+ influx always exceeded the efflux, a coupling between the transport processes across either plasma membrane became evident from the time-courses of the two fluxes.