Transmembrane ionic movements are thought to occur through special membrane areas or channels. Until recently, it has not been possible to study the properties of individual ionic channels, directly. We have used the extracellular patch clamp technique to resolve unitary sodium channels in enzyme-dissociated rabbit ventricular myocytes. Depolarizing voltage clamp steps elicited rectangular inward current fluctuations representing open-closed transition of the channels. Probability of channel opening increased with depolarization. Single channel current amplitude ranged from 0.6 to 1.9 pA. Channel open time increased with depolarization. Sodium channels could be distinguished from calcium channels by their insensitivity to cadmium and inactivation voltage range. First-event latency histograms suggest multiple-closed states. The patch clamp technique can be readily applied to the study of sodium channels in adult cardiac myocytes.