Chronic variations in load are clinically relevant as mechanical overload can result in heart failure and unloading with left ventricular assist devices is used in the management of patients with heart failure. Previous studies have investigated the effect of chronic load variation on the electrophysiological properties of the left ventricle using isolated cardiomyocytes and whole hearts. Here, we study the load-dependent remodeling using myocardial slices, an intact multicellular preparation that allows the investigation of functional and structural regional heterogeneities. Lewis rats were anaesthetized with isoflurane and pressure overload was induced by transverse aortic constriction (TAC) for 10 and 20 weeks. Sham-operated animals (S) served as control. Mechanical unloading (MU) was obtained by heterotopic abdominal heart-lung transplantation for 8 weeks. The recipient’s native heart acted as control. Vibratome-cut myocardial slices (350µm thick) were prepared from the left ventricle, tangentially to the epicardial surface. Slices were electrically stimulated and studied using a multi-electrode array (MEA) system. Extracellular field potentials, recorded from 60 MEA microelectrodes, were analysed to measure field potential duration (FPD) – an index of action potential duration. Conduction velocity (CV) was also measured. Values are mean ± S.E.M., compared by t-test. The measurement of heart weight/body weight ratio confirmed the presence of hypertrophy in the TAC group (10 weeks: 4.9±0.2, n=5; S: 3.8±0.1, n=5; p<0.01, 20 weeks: 4.1±0.2; n=4; S: 3.1±0.1 n=5; p<0.05) and atrophy in the unloading group (MU: 1.4±0.1, n=3; control: 3.5±0.1, n=3; p<0.05). Myocardial slices prepared from the TAC group showed significantly faster CV compared with the SHAM group at 10 weeks (TAC: 49±4 cm/s, n=25 slices; S: 35±4 cm/s, n=17 slices; p<0.05) but no significant change at 20 weeks (TAC: 67±10 cm/s, n=17 slices; S: 56±6 cm/s, n=22 slices; p>0.05). FPD did not change at 10 weeks TAC (TAC: 124±3 ms, n=16 slices; S: 122±2 ms; n=13 slices; p>0.05), but was significantly prolonged at 20 weeks (TAC: 122±2 ms, n=18 slices; S: 114±3 ms, n=20 slices; p<0.05). Slices obtained from the MU group showed significantly prolonged FPD (MU: 116±2 ms, n=11 slices; control: 108±2 ms, n=13 slices; p<0.05), but unchanged CV (MU: 30±3 cm/s, n=10 slices; control: 32±5cm/s, n=13 slices; p>0.05). Our data show that chronic mechanical overloading temporarily increases CV with delayed repolarisation. The latter is also observed during unloading without effects on CV. Delayed repolarisation may represent a compensatory response to variations in mechanical load and underlie possible arrhythmogenic effects. Future studies will employ myocardial slices to determine the structural alterations responsible for the observed electrophysiological remodeling.