The atrioventricular node (AVN) is normally the only site at which electrical activity can pass from the atria to the ventricles of the heart, so that changes in AVN conduction may be pro-arrhythmic or protective. Previous work has shown that acidosis, which occurs in a number of clinical conditions, slows the rate of action potential conduction through the node (Aberra et al., 2001; Walker et al., 2004). However, although conduction depends critically on ionic currents, little is known about the effects of acidosis on the electrophysiology of the AVN. The aim of the present study was to investigate the effect of acidosis on membrane currents recorded from myocytes isolated from rabbit AVN as described previously (Hancox et al., 1993). Ionic currents were recorded at 37°C using the whole-cell patch-clamp technique. Acidosis was produced by reducing the pH of the HEPES buffered bathing solution from pH 7.4 to pH 6.3; intracellular [Ca²⁺] and pH were buffered by inclusion of 5 mM BAPTA and 10 mM HEPES respectively in the pipette solution. Membrane potential was held at -40 mV and test pulses applied to a range of voltages between -120 and +50 mV at 5 s intervals. AVN cells could be distinguished from atrial and ventricular cells on the basis of size, small or absent I_K1 and high membrane resistance (Hancox et al., 1993): AVN cells had a membrane capacitance of 34.8±1.6 pF (mean±SEM, n=29) and input resistance of 681.8±60.4 MΩ at -40 mV (the approximate zero-current potential). I_Ca and I_Kr, identified based on previous work (Howarth et al., 1996), and I_f were recorded. Acidosis decreased the amplitude of I_Ca at all potentials (from 17.7±1.8 to 12.5±1.4 pA/pF at 0 mV; p<0.01, paired t-test, n=29), and increased its time to peak (from 3.8±0.3 to 4.8±0.4 ms at 0 mV; p<0.01, n=29). Acidosis also decreased I_Kr tail current (from 1.7±0.1 to 1.1±0.2 pA/pF at +20 mV; p<0.01, n=19). In 7 cells that exhibited I_f, this current was unaffected by acidosis (-2.7±0.7 pA/pF at pH 7.4; -2.7±0.7 pA/pF at pH 6.3, recorded at -120 mV; p>0.05). We conclude that I_f is unlikely to be involved in the response of the AVN to acidosis, but that inhibition of I_Ca and I_Kr by acidosis is likely to play a significant role in action potential repolarisation and conduction in the AVN.