Objective To investigate the mechanism of ventricular dilation-induced arrhythmias by dilating isolated rat hearts. Methods Isolated rat hearts were perfused by Langerdorff method. After equilibration, 80 hearts were randomly divided into four groups as follows: (1) control group (n=20), (2) Ca2+ preconditioning (CPC) group (n=20), (3) streptomycin group (n=20), and (4) CPC + streptomycin group (n=20). A latex balloon which can be filled with fluid was anchored in the left ventricle through the left atrium and mitral valve. Epicardial ECG of the left ventricle, left ventricular pressure, coronary flow and heart rate were recorded before and during ventricular dilation by injecting fluid into the latex balloon. The rate and duration of ventricular dilation-induced arrhythmias were recorded. Results Under the same increase in ventricular end-diastolic pressure made by inflation of the balloon, the rate of arrhythmias was 100% and duration of arrhythmias was 2. 56±0.46s in the control group. Both the rates of premature ventricular beat (90%)and ventricular tachycardia 70%)were high. Compared with the control group, the total rate (60%) of arrhythmias was lower, and duration(1.67±0.61s)of arrhythmias was shorter in the CPC group. Both the rates of premature ventricular beat (60%)and ventricular tachycardia (40%) were low comparatively. The rate of arrhythmias (45%)was lower and duration (1.64±0.42s)of arrhythmias was shorter, and the rates of premature ventricular beat (30%) or ventricular tachycardia (35%)were lower in the streptomycin group than in the control one. The least ventricular dilation-induced arrhythmias occurred in the CPC + streptomycin group. The rate of arrhythmias(10%)was the lowest and duration (1.01±0.37s)of arrhythmias was the shortest; both the rates of premature ventricular beat(5%) and ventricular tachycardia (10%)were the lowest. Conclusions Ventricular dilation may induce arrhythmias in isolated rat hearts. Stretch-activated ion channel and the increase in [Ca2+]i are supposed to play important roles in the pathological mechanism.