Ultrathin lipidic membranes containing one ten-thousandth of a special proteolipid from electric organ of Electrophorus reacted to the addition of acetylcholine by a rapid and transient increase in conductance. Such a change was not induced by choline and is greatly reduced by a previous application of d-tubocurarine. These properties, resembling those from chemically excitable membranes, were not observed with another proteolipid from the same tissue.