Lipid membranes made of lecithin and cholesterol were formed by self-assembly in a small aperture on an agar support. The membranes exhibited an average electric resistance of 135 Gfl and a capacitance of 0.43 µF/cm2. Gramicidin, known to form a channel in uni-lamellar lipid bilayers, reduced the electric resistance to a Mf level, thus showing the membranes to be of a uni-lamellar bilayer type. The membrane stability was investigated against perturbation with electric potentials and against mechanical agitation in the contacted aqueous solution. About 80% of the membrane preparations remained intact after applying electric potentials of between +1500 mV and -1500 mV. A similar percentage of the membranes stayed intact under 100 rpm magnet stirring in a 30 ml vessel. Membranes containing valinomycin responded to K+ ions with changes in both the membrane conductance and the membrane potential.