The present study was conducted to identify the effect of vasopressin (AVP) on electrocardiographic changes produced by ischemia-reperfusion. Male rats were divided into seven groups (n=8-13) subjected to 30min ischemia and 120 min reperfusion. In protocol I (control group), saline was administered before ischemia. In protocol II, different doses of AVP (0.015, 0.03, 0.06 and 0.12μg/rat) were infused 10 min before ischemia. In protocol III SR49059 (1 mg/kg), was injected 20 min prior to ischemia with and without the effective dose of AVP (0.03 g/rat). Ischemia-induced arrhythmia and myocardial infarct size (IS) were measured. Different doses of vasopressin decreased IS. There were no significant differences in PR, QRS duration and &DGR;T/amp;DGR;ST ratio between control and intervention groups in ischemia. ST elevation was significantly increased in control and AVP 0.015, 0.03, 0.06 groups during ischemia. In AVP 0.12 group there was no significant difference in ST deviation between the baseline and ischemia phase. JT interval was significantly increased in control and antagonist group during ischemia. AVP 0.12μ/rat prevented the increase of JT interval in ischemia compared to the baseline. In summary, AVP mediated preconditioning improved ST resolution, prevented prolongation of JT interval and decreased the likelihood of subsequently ventricular arrhythmia.