OBJECTIVE To examine the hypothesis that an altered left ventricular depolarization sequence may augment infarct size. METHODS Twenty-one New Zealand male rabbits were anesthetized and ventilated. The chest was opened and two electrodes were placed on the right atrium and ventricle. The rabbits were then randomized to atrial (n = 7), atrioventricular (AV) sequential (n = 7) or no (n = 7) pacing. The pacing rate was 20 beats/min higher than the sinus rate. After 1 min of pacing, the left coronary artery was occluded by a snare. After 30 mins, the snare was released and pacing was stopped. After 120 mins of reperfusion the experiment was terminated. Normal areas and areas at risk were delineated, infarct size was measured and the infarcted areas and areas at risk were planimetered. RESULTS All results were expressed in cubic centimetres, and the ratio of the infarcted area to area at risk was calculated as a percentage (%I:R). The double product during ischemia was 21,546 +/- 2300 in controls, 23,000 +/- 3005 in rabbits with atrial pacing and 24,418 +/- 4253 in rabbits with AV pacing (F = 1.33, P = 0.28), and %I:R was 41.4 +/- 19.8, 43.9 +/- 15.4 and 38.4 +/- 18.4 (F = 0.16, P = 0.84), respectively. CONCLUSIONS An altered left ventricular depolarization sequence in rabbit hearts does not increase infarct size.