The atrioventricular node (AVN) plays a critical role in normal cardiac conduction and can also take over pacemaking of the ventricles should the sinoatrial node fail. The cellular electrophysiological basis of AVN pacemaking is incompletely understood, although it is likely to involve multiple ionic conductances (Hancox et al., 2003). This study aimed to characterise for the first time sodium-dependent background inward current in rabbit AVN cells. Adult male New Zealand White rabbits were killed in accordance with UK Home Office legislation and AVN cells were isolated as described previously (Hancox et al., 1993). Whole-cell voltage-clamp using step and ramp protocols was used to record the background current, after the Ca2+, K+, Na+-K+ pump and Na+-Ca2+ exchange currents were inhibited by appropriate blockers (Hagiwara et al., 1992). External solutions were as follows: 150mM-Na+ (mM): 150 NaCl, 5 HEPES, 2 CsCl, 2 NiCl2, 1 BaCl2, 1 MgCl2, 0.01 Strophanthidin; Tris Na+-free: Na+ replaced with equi-molar Tris-hydrochloride. The pipette solution contained (mM): 120 CsOH, 20 CsCl, 5 HEPES, 10 EGTA, 5 K2-creatine phosphate, 5 Mg-ATP, 2 MgCl2, 100 Aspartic acid. Changing the bath solution from Tris Na+-free to 150 mM-Na+ induced an inward current, and the slope conductance at -50 mV increased from 0.54±0.03 to 0.91±0.05 nS (mean±S.E.M; n=61 cells). The Na+-dependent inward background current density obtained by subtracting the current in Tris Na+-free from that in 150 mM-Na+ solution was -0.82±0.05 pA/pF at -50 mV (n=61). When a range of external Na+ concentrations ([Na+]o) between 30-200 mM was applied, the background current amplitude varied in proportion to [Na+]o (n=8). When external Na+ was replaced with various monovalent cations, the background current (measured at -100 mV) amplitude increased in the order: Li+<Na+<Cs+<K+<Rb+ (n=7). The Na+-dependent background current was partially inhibited by cation channel blocker SKF-96365 (10 µM produced ~32.1±5.3% inhibition at -100 mV; n=8, P<0.01). We conclude that a Na+-dependent background inward current exists in rabbit AVN cells and that the channels mediating this current exhibit poor cation selectivity.