1. The effects of tetraethylammonium (TEA) on the membrane activity of the antral circular muscle of the guinea-pig stomach were investigated with micro-electrode and double sucrose gap methods.2. In a concentration of 1-1.5 x 10(-3) g/ml. (3-5 mM), the membrane potential was not influenced; the membrane resistance measured by inward current pulses remained the same but the rectifying property of the membrane was suppressed.3. TEA (1-1.5 x 10(-3) g/ml.) enhanced the spike amplitude markedly even from fibres which generated graded responses.4. TEA (1-1.5 x 10(-3) g/ml.) did not increase the maximum rate of rise of the spike but decreased the maximum rate of fall of the spike markedly.5. In Na-free (Tris or sucrose) solution, in K-deficient and excess-K solutions, TEA (1-1.5 x 10(-3) g/ml.) suppressed the rectifying property of the membrane and enhanced the spike amplitude.6. Atropine (10(-6) g/ml.) had no effect on the enhancement of the spike amplitude produced by TEA.7. The minimum concentration of Ca ions required for the effect of TEA on the spike amplitude was one fifth of the normal concentration. TEA also enhanced the spike amplitude in Sr-Krebs.8. The possible role of TEA on the membrane activity is considered to be due to suppression of the K conductance when the membrane is depolarized. Alternative possible roles of TEA on the spike amplitude are also discussed.