Patch clamp technique was employed to record single Na channel current in isolated guinea-pig ventricular myocytes. Four types of late Na channel currents were observed with whole cell and inside-out recording, which are (1) isolated brief openings, (2) scattered openings, (3) long openings and (4) bursts. The occurrence frequencies of the four types were different. Bursts appeared about 1 in every 2,000 depolarizations, with an open time constant longer than others. However, all the four types of late current openings, as well as the early openings occurring immediately upon depolarization, could be blocked by tetrodotoxin. It was also found that at a certain stimulation frequency, normal action potential duration (APD) was not invariable as supposed before, but varied from stimulation to stimulation. Special Na channel blocker, TTX, shortened the APD and refractory period and abolished the dispersion. The above results suggest that the activities of the various types of late Na channel play an important role in the formation of action potential plateau.