Atrial and ventricular myocytes were exposed to Ca2+- and K+-free solutions containing blockers of gated channel and exchange currents. Replacement of external sodium by large organic cations revealed a background sodium current ib, Na. In atrial cells, the average conductance was 5.0 pS pF-1. In ventricular cells the conductance was 2.3 pS pF-1. Together with previous results, these figures reveal a strong gradient of background current density: sinus > atrium > ventricle. Replacement of sodium with inorganic cations showed that the channel selectivity behaves like an Eisenman group III/IV sequence, in agreement with previous results. The permeability of the channel to TMA was found to be pH dependent, suggesting that protonation of the channel is a factor determining permeation in addition to ionic size. The values of gb, Na obtained from these experiments are very similar to those assumed in computer modelling of cardiac cell electrical activity.