Single-channel sodium currents that could be blocked with TTX were elicited by depolarizing voltage pulses in either cell-attached or inside-out patches from rat ventricular myocytes. A transient burst of channels was followed by late-opening (persistent) channels with low open probability. Conditioning depolarizing pre-pulses that inactivated transient channels and `chattering' late-opening channels had no effect on persistent channels. The open probability of persistent channels reached a maximum at more negative potentials than transient channels. Between -70 mV and -40 mV, the average open time of persistent channels increased, whereas the average open time of transient channels did not change significantly, so the open times of the two channels diverged as the potential became more positive. The conductance of transient and persistent channels was similar, and the conductance of both kinds of channel increased at more depolarized potentials.