In this paper we report the direct measurement of rare Na channel events that occur during the cardiac action potential, viz., channels that open at the upstroke and remain open throughout the plateau and early repolarization phase. The technique we use allows us to record channel activity and action potentials at the same time; thus, we are certain of when the Na channels open and when they finally close. The slow Na channels have the same voltage dependence, single-channel conductance, and TTX sensitivity as the fast Na channels, and they conduct Li. It therefore seems likely that the fast and the slow currents flow through the same channel. If this interpretation is correct, then the Na channel not only initiates the action potential but also helps to maintain its plateau. It is possible that the slow Na currents represent a separate collection of channels rather than a low-probability state of the fast Na channels. Regardless of which interpretation is correct, the present experiments allow us to assess the effect of the slow currents on action potential shape and on sustained Na entry.