We used the patch clamp technique to study the nature of the late sodium current in guinea pig ventricular myocytes. In a cell attached mode of single channel recording at room temperature (22-24°C), two kinds of late (100 msec or more after beginning of the depolarizing pulse) sodium channel activities were recognized. One is isolated brief openings appearing once for about 120 depolarizations per channel (background type), while the other type is sustained openings with rapid interruptions (burst type) that occurred only once for 2,700 depolarizations per channel. The time constant obtained from the open time histogram of the burst type (1.05 msec) was about five times longer than that of background type (0.18 msec, measured at the potential 10 mV above the threshold). Magnitude of the late sodium current flowing through the entire surface of a myocyte was estimated with tetrodotoxin (60 μM), a specific inhibitor of sodium channels, in whole-cell clamp experiments. The steady tetrodotoxin-sensitive current of 12 to 50 pA was registered at -40 mV (26±14 pA, mean±SD, n=5), in good agreement with the late sodium current calculated from the single channel recording. Tetrodotoxin produced small (-10percent;) but significant decreases in the action potential duration. These results suggest the presence of a small but significant late sodium current with slow inactlvation kinetics and that this current probably plays a significant role in maintaining the action potential plateau and the duration in guinea pig ventricular myocytes.