Recent evidence implicates Ca2+ release from the sarcoplasmic reticulum (SR) in pacemaking in the cardiac atrioventricular node (AVN; Nikmaram et al., 2008; Ridley et al., 2008). In order to influence pacemaking, SR Ca2+ release must influence sarcolemmal ionic currents. Accordingly, the aim of the present study was to investigate the effect of the sodium-calcium exchange (NCX) inhibitor KB-R7943 (Kimura et al., 1999) on the spontaneous activity of 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). Spontaneous action potentials (APs) were recorded continuously by current clamping with gap-free acquisition. NCX current (INCX) was measured as 5 mM Ni2+-sensitive current elicited by ramp voltage commands from +60 to -80 mV with major interfering currents blocked (Convery & Hancox, 2000); calcium current (ICa,L) was elicited using a test pulse from -40 to 0 mV at 0.2 Hz, with a caesium-based pipette solution. All recordings were performed at 37 °C. Rapid application of 5 µM KB-R7943 to spontaneously beating AVN cells caused spontaneous APs to cease or to give way to oscillations of membrane potential within 40 s in 6 out of 14 cells. At 40 s of exposure, and incorporating measurements from all cells studied, the AP rate decreased from 3.99±0.47 to 1.87±0.48 s-1 (Mean±SEM, n=14, P<0.05, paired t-test); the slope of pacemaker diastolic depolarization decreased from 113.9±13.3 to 32.4±8.8 mV s-1 (n=14, P<0.01). KB-R7943 also reduced AP overshoot and maximum diastolic potential. 5 µM KB-R7943 blocked 95±14% of inward INCX recorded at -60 mV (n=5) and reduced peak ICa,L at 0 mV by 31±4% (n=12). We conclude that the NCX inhibitor KB-R7943 inhibits spontaneous activity of AVN cells and that this inhibition may involve blockade of both INCX and ICa,L.