['Postresuscitation myocardial dysfunction is one of the leading causes of early death after initial success of resuscitation, the mechanisms of postresuscitation myocardial dysfunction remain controversial. We hypothesize that ischemia injury, rather than reperfusion injury is the major cause of postresuscitation myocardial dysfunction. We proposed to investigate the separate effects of ischemia and reperfusion injury on postresuscitation myocardial dysfunction.', 'Thirty-three Langendorff-perfused isolated rat hearts were subjected to 15 min of global ischemia followed by 120 min of reperfusion. Pentazocine was utilized as a myocardial protective agent, either before ischemia or during reperfusion. All hearts were randomized into 3 groups: (1) "ischemia protection", in which pentazocine was infused 10 min prior to global ischemia, (2) "reperfusion protection", in which pentazocine was infused during 2h of reperfusion and (3) "control", with no pentazocine infusion. Left ventricular (LV) functions were measured by the maximal rate of LV pressure rise (dP/dt(max)) and decline (-dP/dt(max)), the maximal LV diastolic pressure (LVDP). The incidences of postischemic arrhythmias were measured.', 'When pentazocine was administered before onset of ischemia, the LV systolic and diastolic functions were significantly greater, and the postischemic arrhythmias were significantly less in comparison to those with reperfusion protection (p<0.05) and the control group (p<0.05).'] Does opioid receptor agonist reduce myocardial ischemic injury when administered during early phase of myocardial ischemia?