Acute myocardial ischaemiaand reperfusion (I-R) are major causes of ventricular arrhythmias in patients with a history of coronary artery disease. Ursodeoxycholic acid (UDCA)has previously been shown to be antiarrhythmic in fetal hearts.This study was performed to investigate if UDCA protects against ischaemia-induced and reperfusion-induced arrhythmias in the adult myocardium,43andcomparesthe effect of acute (perfusion only) versus prolonged (2 weeks pre-treatment plus perfusion) UDCA administration. Langendorff-perfused adult Sprague-Dawley rat hearts were subjected to acute regional ischaemia by ligation of the left anterior descending artery(10 minutes), followed by reperfusion (2 minutes), and arrhythmia incidence quantified. Prolonged UDCA administration reduced the incidence of acute ischaemia-induced arrhythmias (p=0.028),with a eduction in number of ventricular ectopic beats during the ischaemic phase compared with acute treatment(10±3 vs 58±15, p=0.036).No antiarrhythmic effect was observed in the acute UDCA administration group. Neither acute nor prolonged UDCA treatment altered the incidence of reperfusion arrhythmias.The antiarrhythmic effect of UDCA maybe partially mediated by an increase in cardiac wavelength, due tothe attenuation of conduction velocity slowing (p=0.03), and the preservation of Connexin43 phosphorylation during acute ischaemia(p=0.0027). The potential antiarrhythmic effects of prolonged UDCA administration merit further investigation.