A counter flow of electrolyte can be applied successfully for the separation of large samples at high concentration. The effect of a counter flow of electrolyte on the profile of the zone boundary has been studied. Disturbances are caused mainly by the hydrodynamic flow of the electrolyte, as shown by the application of two different viscosities of the leading electrolyte. The disturbances were recorded with a conductivity detector and a photographic procedure (dyes were applied in combination of uncoloured spacers). If the effective mobilities of two components differ only slightly, a 100% counter flow of electrolyte (which is the hydrodynamic flow that is in equilibrium with the "electrophoretic flow") prevents the formation of sharp zone boundaries. Small counter flows of electrolyte, on the other hand, sharpen the zone boundaries and can therefore be usefully applied. A new type of membrane pump is described for experiments with a regulated counter flow of electrolyte.