The aim of this paper was to improve operative outcome during open-heart surgery in patients with failing hearts, the composition of cardioplegic solutions has to be further optimized. HTK-N46b, a novel cardioplegic solution, has been developed for efficient protection of the energy state of myocytes as well as endothelial cells. Aim of this study is the evaluation of HTK-N46b in comparison to its precursor Custodiol® (HTK) in failing rat hearts undergoing ischemia/reperfusion. In male Sprague Dawley rats myocardial infarction (MI) was induced by LAD ligation. Six weeks after MI cardiac function was determined by transthoracic echocardiography. Sixteen animals with hearts showing a fractional shortening <25% were randomly assigned to two groups, HTK (N.=8) and HTK-N46b (N.=8). After excision hearts were evaluated in an erythrocyte-perfused isolated working heart model. Cold ischemia (4°C) for 60 minutes was followed by 45 minutes of reperfusion. Cardiac arrest was induced either with HTK or HTK-N46b at the beginning of ischemia. At similar preischemic fractional shortening (HTK-N46b: 14.41±1.83% vs. HTK: 14.91±1.92%; NS) postischemic recovery of stroke volume and stroke work were significantly improved in the HTK-N46b rat hearts compared to HTK. Concerning recovery of coronary flow there was no difference between groups. At the end of reperfusion the HTK-N46b protected group revealed higher levels of ATP (HTK-N46b: 22.01±0.89 nmol/mg protein vs. HTK: 16.83±1.72 nmol/mg protein; P<0.05) and energy charge (HTK-N46b: 0.82±0.02 vs. HTK: 0.74±0.02; P<0.05). HTK-N46b showed superior cardioprotective properties according to postischemic hemodynamic recovery and biochemical markers compared to HTK in failing rat hearts.