Stretch of the left ventricular myocardium in isolated hearts is commonly achieved by inflation of an indwelling fluid-filled balloon. This stretch can trigger arrhythmias, often explained in terms of the activation of mechanosensitive ion channels within the myocardium. However, Purkinje fibres are also known to be mechanosensitive and free-running Purkinje fibres line the endocardial surface potentially in direct contact with any balloon. Brief exposure to Lugol’s solution (Lugol) has previous been reported to destroy surface Purkinje fibres whilst leaving the underlying myocardium intact (e.g. Dosdall et al, 2008). The aim of our study was to test whether stretch-activated arrhythmias were sensitive to Lugol and thus implicate Purkinje fibres in their origin.Wistar rats (N = 4, 250-280g) were humanely killed, hearts were rapidly removed and Langendorff perfused with a HEPES buffered physiological saline at 38C. An inflatable cellophane balloon (connected to a pressure transducer) and a fine tube (OD, 0.9mm) were placed in the left ventricle and secured in place by tying to the aortic cannula. Needle electrodes were used to record the ECG in lead II mode. The left ventricle was stretched by inflation of the balloon to 100µl over approx. 2s after washing of the endocardium with either 1ml of saline or 0.1ml Lugol (2g KI, 1g I2 in 100ml H20) followed by 0.9ml saline. The endocardium was exposed to Lugol for 10s. Endocardial wash with Lugol did not affect sinus rhythm or left ventricular developed pressure (P > 0.05, paired t-test). Following endocardial wash with either saline or Lugol, left ventricular stretch significantly increased the standard deviation of the beat to beat interval (SDNN) but the increase was significantly attenuated following Lugol treatment compared to saline (SDNN pre-stretch saline 3.5 ± 1.6 ms vs pre-stretch Lugol 3.3 ± 0.5 ms P > 0.05, post-stretch saline 113.8 ± 15.1 ms vs post-stretch Lugol 39.9 ± 14.5 ms P < 0.001, pre-post comparison, saline P < 0.001, Lugol P = 0.02, mean ± SEM, 2-way RMANOVA n = 2 saline and 2 Lugol stretches in each of 4 hearts).The lack of effect of brief endocardial exposure to Lugol on either sinus rhythm or left ventricular developed pressure indicates a local, endocardial site of action. We conclude that stretch-induced arrhythmias provoked by balloon inflation are at least in part triggered by mechanical stimulation of free-running Purkinje fibres. The mechanism of this stretch-activation is under study and may be relevant to the investigation and treatment of arrhythmias associated with Purkinje fibres.