1. Effects of membrane polarization and of reduction in external K and Cl concentration on the inhibitory potential were investigated in the guinea-pig taenia coli.2. Depolarization of the membrane increased the inhibitory potential while hyperpolarization decreased it. The relationship between the degree of membrane polarization and the amplitude of inhibitory potential was linear. The inhibitory potential was abolished or slightly reversed in polarity, when the membrane was hyperpolarized by 25-40 mV in different preparations.3. Removal of external K ion depolarized the membrane for about 5 min and increased the inhibitory potential more than could be accounted for by the depolarization. Readmission of K transiently hyperpolarized the membrane, probably due to an activation of the Na-K pump, and reduced the inhibitory potential, but no reversal of polarity in the inhibitory potential was observed during this hyperpolarizing phase.4. The membrane was transiently depolarized when the external Cl concentration was reduced by substituting with isethionate. Hyperpolarization was produced by restoring the external Cl concentration to normal. Changes in the amplitude of inhibitory potentials during alterations in Cl concentration occurred as expected from the shift of the membrane potential.5. From the results, it is concluded that the membrane conductance is increased during the inhibitory potential, and that an increase in the K permeability is the main factor for hyperpolarization of the membrane.