In the heart, the ventricular conduction system comprises an intricate network of Purkinje fibres (PFs) that conducts electrical activity from the atrioventricular node to the ventricular myocardium and activates coordinated ventricular contraction. Conduction abnormalities associated with the ventricular conduction system are common in heart failure (1) and such patients often require cardiac resynchronisation therapy. In a rabbit model of congestive heart failure (CHF), we have investigated electrical and molecular remodelling in PFs.Three month old male New Zealand white rabbits (~3 kg) were anaesthetised with ketamine intramuscular injection and isoflurane inhalation. CHF was induced (n=10) by combined volume and pressure overload caused by surgical destruction of aortic valve (week 0) and banding of abdominal aorta (week 3) respectively. CHF characteristics were confirmed by echocardiography prior to termination at week 8. Ten sham operated animals were used as controls. In left ventricular (LV) preparations, intracellular action potentials were recorded using microelectrodes from LV myocardium and three regions of the ventricular conduction system: left bundle branch, free-running PFs and junctional PFs (at Purkinje-ventricular junction). In free-running PFs and LV myocardium, mRNA expression of ~30 ion channels was measured using qPCR. This work was carried out in accordance with UK Home Office regulations.In CHF rabbits, echocardiography showed significant LV hypertrophy, reduced LV fractional shortening (25.5±3.2 vs. 41.3±1.4 % in control; P<0.05) and increased diastolic LV internal diameter (2.2±0.1 vs. 1.5±0.03 cm in control; P<0.05). In anaesthetized CHF rabbits, the ECG RR-interval was 228±18 ms, similar to control. In the CHF group, the left bundle branch action potentials showed a slightly increased maximum upstroke velocity (dV/dtmax), amplitude and action potential duration, but the differences were not significant. Action potential duration was prolonged in free-running PFs and junctional PFs by 84% and 81%, respectively, in the CHF cohort. In agreement with this observation, in free-running PFs, we found downregulation of mRNA responsible for major repolarizing K+ channels: Kv1.5 (responsible for IK,ur), KChIP2 (responsible for Ito), ERG (responsible for IK,r) and KvLQT1 (responsible for IK,s). Action potentials recorded in the LV myocardium showed a 43% increase in dV/dtmax in the CHF group, consistent with an upregulation of Nav1.5 mRNA that is responsible for INa and the action potential upstroke.In CHF, we observe ion channel remodelling and action potential changes that are heterogeneous within the ventricular conduction system. LV dilatation and consequent stretching of the PFs are likely to contribute to the marked changes observed in the free-running PFs.