The effects of changes in extracellular Na⁺ and Ca⁺ concentration on the membrane potential and contractility were studied in the antral circular muscle of guinea pig stomach in order to elucidate the existence and the nature of Na⁺/Ca²⁺ exchange mechanism. All experiments were performed in tris buffered Tyrode solution which was aerated with 100% O² and kept at 35℃. The treatment of 10^-5 ouabain was performed to induce intracellular Na⁺ loading prior to the start of experiment. The results were as follows: 1. Na⁺-free Tyrode or high Ca²⁺-Tyrode solution hyperpolarized the membrane potential and induced contracture. The time course of contracture was similar to that of change in membrane potential. 2. The degree of hyperpolarization and the amplitude of contracture decreased in accordance with the increase of extracellular Na⁺ concentration. 3. Na⁺-free contracture was developed even after blocking the influence of intrinsic nerves by the pretreatment with atropine, guanethidine and TTX. 4. Ca²⁺-channel blockers(D-600 or Mn²⁺) and the blocker of intracellular Ca²⁺ release from sarcoplasmic reticulum(ryanodine) did not suppress the development of Na⁺-free contracture. And also, dinitrophenol had no effect on Na⁺-free contracture. 5. Dose-response relationship between extracellular Na⁺ concentrations and the magnitude of contractures showed a sigmoid pattern. The slope of straight line from Hill plot was 2.7. 6. In parallel with the increase of extracellular Ca²⁺ concentration, the amplitude of contracture increased dose dependently and was maximum at 8 mM Ca²⁺-Tyrode solution. 7. The relationship between extracellular Ca²⁺ concentrations and the magnitude of contractures showed hyperbolic pattern. The slope of straight line from Hill plot was 1.1. From the above results, it is suggested that Na⁺/Ca²⁺ exchange mechanism exists in the antral circular muscle of guinea pig stomach and this mechanism affects the membrane potential electrogenically.