Abstract: The effect of rhomotoxin on the transmembrane potential and contractile force was studied in isolated guinea pig papillary muscles. Rhomotoxin produced positive isotropic effect. It decreased resting potential and amplitude of action potential and shortened action potential duration at 20% and 50% repolarization. These effects were shown to be time-and dose-dependent. Tetrodotoxin, a specific Na⁺ channel blocker, shifted the time-and dose-response curves of rhomotoxin and reversed the effects of rhomotoxin. The automaticity induced by rhomotoxin was also abolished by tetrodotoxin almost immediately.The present results suggest that rhomotoxin is an agent that affects myocardial Na⁺ channel selectively and results in an increase of Na⁺ inward flow. Rhomotoxin can be used as a tool in the study of the differentiation of cardiac Na⁺ channels.