Background: Veteran endurance athletes are prone to cardiac arrhythmias such as bradycardia and atrioventricular (AV) block, resulting in increased incidence of pacemaker implantation. Previously we reported that training-induced bradycardia is due to diffuse remodelling of pacemaking ion channels in the sinoatrial node1). Here we examined the electrophysiology and ion channel profile of the AV node (AVN) in response to chronic endurance exercise. Methods: 10 week old C57BL/6j mice were trained by swimming for 60 min/day, 5 days/week for 5 months and compared to control sedentary mice. At the end of the training period, animals were anaesthetised with 2% isofluorane and ventilated following which a thoracotomy was used for rapid atrial pacing with an octapolar electrode catheter. Mice were then killed by cervical dislocation under terminal anaesthesia and AVN preparations rapidly dissected and frozen. Total RNA was isolated from serial 20 μm sections of the AVN and right atrium (RA), identified by HCN4 immunolabelling and collected by laser capture microdissection. Preamplified cDNA was loaded onto TaqMan Low Density Array cards and the expression levels of 96 transcripts including key mediators of the membrane and Ca2+ clock mechanisms of pacemaking were measured by qPCR. Results: Trained mice were bradycardic and had a prolonged PR interval (Trained, 44.0±1.0 ms; Control, 40.0±1.0 ms; P<0.05, n=12-14). In trained mice, in vivo programmed electrical stimulation protocols revealed a significantly increased Wenckebach cycle length (Trained, 99.8±1.4 ms; Control, 85.5±2.1 ms; P<0.001, n=11-14) and prolonged AV node effective refractory period (Trained, 80.1±2.6 ms; Control, 62.6±2.8 ms; P<0.05, n=11-13) suggesting altered conduction and electrophysiological remodelling of the AVN. 43.3% of the genes studied were downregulated in the AVN of trained mice (P<0.05). The trained AVN presented with lower expression of HCN4 (responsible for funny current, If), RyR2 (SR Ca2+ release channel), NCX1 (Na+-Ca2+ exchanger) and Cav3.1 and Cav3.2 (responsible for T-type Ca2+ current, ICa,T). Reduced expression levels of gap junction channels (Cx30.2, Cx40, Cx43) and inflammatory and fibrosis genes (interleukin 1β, NFκB, TGFβ1, Tnfα, collagen type 1 and 3α, fibronectin 1, vimentin) were also observed. These changes might be explained by training-induced alterations in key transcription factors that can exert transcriptional control (Irx3, Klf4, Mef2c, Nkx2.5, Tbx3, Shox2). Only <8.9% of genes were significantly affected in the RA. Conclusions: Chronic endurance exercise results in AVN dysfunction underscored by striking transcriptional remodelling of AVN genes responsible for action potential generation and propagation. Data provide a new paradigm for understanding the occurrence of heart block in veteran athletes.