Isolated rabbit hearts were perfused normoxically or ischemically using the technique of Langendorff. The hearts were perfused with a modified Krebs-Henseleit solution, perturbated with 95% O2/5% CO2 or in case of ischemia with N2 which replaced the O2. The perfusion rate was 25 ml/min under normoxic and 2.5 ml/min under ischemic conditions. The oxygen pressure was about 65 kPa in the normoxic and about 6 kPa in the ischemic medium. Reperfusion of ischemic hearts was realized in some cases normoxically. During the ischemic perfusion and reperfusion LDH was released time depended into the perfusion medium and the aortic inflow pressure increased. The LDH release and the increase of the inflow pressure are strong correlated. Both parameters direct to ischemic myocardial lesions. Amrinone applied as bolus in the start period of ischemia or as an infusion during the ischemic and reperfusion time limited the release of LDH and the increase of the inflow pressure. The results were discussed as a direct protecting effect of amrinone against a myocardial ischemic lesion, which is in correspondence with the improving effects of amrinone on the ischemia caused heart failure of canines in vivo, shown by Campbell et al.