Streaming potential measurements have been carried out on NUCLEPORE polycarbonate membranes with pore diameters between 0.015 and 1.0 mu m, equilibrated in KCl or NaCl solutions. Three types of membrane behavior can be seen from the variation of emf with time. Concentration polarization across the membrane is significant for a membrane with pore diameter of 0.015 mu m. For membranes with pore diameters between 0.015 and 0.6 mu m, the emf rises rapidly towards a stable value when a pressure difference is applied. In membranes with pore diameters between 0.6 and 1 mu m, the high liquid velocity alters the interactions at the wall during the course of measurement, and this is reflected in a decreasing emf with time. Some results obtained by varying pore radius and concentration agree with previous studies. Our results, as well as results reported in the literature, are interpreted in terms of a new theoretical method; the electric work method. Electric work is produced for positive liquid velocity, consistent with momentum production at the membrane wall during the measurement. A relation between the magnitude of layer where momentum transfer takes place and the streaming potential can be applied. Momentum transfer at the membrane wall may extend about 0.1 mu m into the pore.