The competition of ion and water fluxes across gramicidin channels was assessed from the concentration distributions of both pore-impermeable and -permeable cations that were simultaneously measured by double-barreled microelectrodes in the immediate vicinity of a planar bilayer. Because water movement across the membrane led to accumulation of solutes on one side of the membrane and depletion on the other, the permeable cation was not only pushed by water across the channel (true solvent drag); it also flowed along its concentration gradient (pseudo-solvent drag). For the demonstration of true solvent drag, a difference between the bulk concentrations on the hypertonic and the hypotonic sides of the membrane was established. It was adjusted to get equal cation concentrations at both membrane/water interfaces. From the sodium and potassium fluxes measured along with membrane conductivity under these conditions, approximately five water molecules were found to be transported simultaneously with one ion through the channel. In diphytanoyl phosphatidylcholine membranes, a single-channel hydraulic permeability coefficient of 1.6 x 10(-14) cm(3) s(-1) was obtained.