STREAMING potentials, the converse of electro-osmosis and one of the four classical electrokinetic phenomena, are the electrical potential differences frequently observed across charged artificial membranes through which water is being forced. They arise from the fact that if the membrane matrix bears, say, negative fixed charges, the aqueous channels through the membrane will contain more mobile cations than mobile anions. An applied water flow will then sweep out fluid with an excess of cations, and the side of the membrane towards which water is flowing will acquire an electrical potential of a sign opposite to that of the membrane matrix. Recently, streaming potentials were observed across a biological membrane1. This communication describes streaming potentials across the gall-bladder of the rabbit and their significance for the problem of biological water transport.