The effect of acetaldehyde on the membrane potential and the membrane resistance of the guinea-pig taenia coli has been studied with an intracellular recording technique. Application of acetaldehyde in the concentration of between 5 and 20 mM produces reduction or elimination of the spontaneous spike activity and hyperpolarization of the membrane. The amplitude and the rates of rise and fall of the action potential are reduced by the drug. The duration of the spike is slightly prolonged. Acetaldehyde also reduces the amplitude of the evoked action potential produced by externally applied cathodal stimulation in twice hypertonic solution. Electronic potentials induced by anodal current are reduced both in size and time course by acetaldehyde during the stage of hyperpolarization.