Electrokinetic measurements, of streaming potential, were carried out on an excised inside-out patch of the vacuolar membrane of Chara corallina. A water activity gradient was imposed across the patch membrane containing a single K(+) channel by addition of sorbitol to one side. Two different K(+) channels were found in the tonoplast. Their open channel conductance was investigated as a function of KCl concentration. They had a maximal open channel conductance of 247 and 173 pS, and an apparent affinity (K(M)) of 116 and 92 mM, respectively. Single-channel zero-current potentials were determined in the presence of an osmotic gradient, and dilution artifacts were corrected for by addition of valinomycin to the bath. Our results suggest that 29 water molecules were coupled to the transport of one K(+) ion in the large conductance K(+) channel which has a pore radius of approximately 1.5 nm.