We measured the membrane potential and the streaming potential of the cellulose acetate membrane in aqueous NaCl solutions at 34 degrees C. The Donnan potential and zeta-potential obtained were negative in sign. The magnitude of the Donnan potential was about two orders higher than that of the zeta-potential in the absolute value for the NaCl solution of 0.1 mol m(-3). We discussed the reason why the Donnan potential and the zeta-potential showed quite different magnitudes. The Donnan potential is related to the charge density expressed in the volume density, while the zeta-potential is related to the charge density expressed in the surface density. Since the effective membrane charge density expressed in volume density increases with the decrease in the pore radius, the magnitude of the Donnan potential depends on the pore radius. Then, for the cellulose acetate membrane having extremely small pore radius, the Donnan potential is larger than the zeta-potential, This indicates that, if the pore size is small enough, the membrane may have the effective charge density that is large enough to affect the partition of ions, even if the zeta-potential is small.