1. The tight seal, whole-cell, voltage-clamp technique was used to record currents from single ventricular myocytes acutely dissociated from adult rat hearts. Subtraction of currents recorded in the presence and absence of tetrodotoxin (TTX, 50 microM) revealed a small, persistent, inward current following a much larger, transient, inward current. 2. Both currents were sodium currents because they reversed close to the sodium equilibrium potential and were depressed when choline was substituted for extracellular sodium. 3. The persistent sodium current could be recorded when the transient current had been inactivated with conditioning depolarization. Only slight inactivation of the persistent current occurred during depolarizing pulses lasting up to 900 ms. 4. A lower concentration of TTX (0.1 microM) blocked the persistent sodium current while having little effect on the transient sodium current. 5. The persistent sodium current was activated at more negative potentials than the transient sodium current. It cannot have been a window current because it was recorded at positive potentials when the transient current was completely inactivated. 6. Because the persistent and transient sodium currents had a different voltage dependence and sensitivity to TTX, it was concluded that different channels are responsible for the two currents.