In patients, loss-of-function mutations of the cardiac sodium channel gene, SCN5A, have been associated with a wide range of arrhythmias including bradycardia, atrioventricular conduction delay, and ventricular fibrillation (Papadatos et al. 2002). The pathophysiological basis of these conditions has not been fully understood. We examined cardiac conduction properties in mice with a targeted disruption of the cardiac sodium channel gene. Adult wild-type mice and mice with a targeted disruption of the cardiac sodium channel gene Scn5a (Scn5a+/-) were humanely killed. The hearts were removed and were Langendorff-perfused with Tyrode solution at a flow rate of 3.8 ml/min at 37°C. Electrocardiogram was first recorded on Langendorff-perfused hearts by custom-made electrodes to measure P-P (representing cycle length and heart rate) and PR interval (representing intraatrial and atrioventricular nodal conduction). A preparation containing the sinoatrial node, atrio-ventricular node and part of the right atrium and ventricle was dissected. The preparation was then fixed in a tissue bath and superfused with Tyrode solution at 37°C at a rate of 4 ml/min through a heat exchanger into chamber. Extracellular potentials (ECPs) were recorded by two bipolar electrodes from the preparations as described by Lei et al. (2004). Data are expressed as means ± S.E.M. (number of preparations or cells). Differences were evaluated by Student’s unpaired t test with a P value <0.05 considered significant. Scn5a+/- hearts showed depressed heart rates (260 ±25 beats/min, n=6) and frequent atrioventricular (A-V) block compared with wild type (WT) hearts (335 ± 19 beats/min, n=6, P<0.05 vs WT). Scn5a+/- preparations exhibited significantly longer sinoatrial conduction time (SACT: 11 ± 2 ms, n=6, P<0.01 vs WT) and sino-ventricular conduction time (S-VCT: 13.6 ± 1.0 ms, n=6, P<0.05 vs WT) compared with WT preparations (SACT: 5 ± 1 ms; S-VCT: 10.7 ± 1.6 ms). Applications of TTX at 1 and 5 μM prolonged SACT by 22 ±7 and 39 ± 4%, respectively, in WT preparations (n=5) and by 30 ± 5 and 56 ± 9% (n=5, P<0.01 vs WT) in Scn5a+/- preparations, suggesting Scn5a+/- preparations are more sensitive to TTX. The results show that Scn5a is involved in normal sino-atrial and atrio-ventricular conduction and reduction of sodium channels can cause cardiac conduction system dysfunction.