The cardioprotective effects of KR-31762, a newly synthesized $K^+_{ATP}$ opener, were evaluated in rat models of ischemia/reperfusion (I/R) heart injury. In isolated rat hearts subjected to 30-min global ischemia followed by 30-min reperfusion, KR-31762 (3 and 10 ${mu}M$) significantly increased the left ventricular developed pressure (LVDP) and double product (heart rate ${ imes}$ LVDP) after 30-min referfusion in a concentration-dependent manner, while decreasing the left ventricular end-diastolic pressure (LVEDP). KR-31762 also significantly increased the time to contracture (TIC) during ischemic period (20.0, 22.4 and 26.4 min for control, 3 and 10 ${mu}M$, respectively), while decreasing the release of lactate dehydrogenase (LDH) from the heart during 30 min reperfusion (30.4, 14.3 and 19.7 U/g heart weight, respectively). All these parameters except LDH release were reversed by glyburide (1 ${mu}M$), a nonselective blocker of $K^+_{ATP}$ channel, but not by 5-hydroxydecanoate, a selective blocker of $mitoK^+_{ATP}$ channel. In anesthetized rats subjected to 45-min occlusion of left anterior descending coronary artery followed by 90-min reperfusion, KR-31762 significantly decreased the infarct size (60.8, 40.5 and 37.8% for control, 0.3 and 1.0 mg/kg, iv, respectively). KR-31762 slightly relaxed the isolated rat aorta precontracted with methoxamine ($IC_{50}:;23.5;{mu}M$). These results suggest that KR-31762 exerts potent cardioprotective effects through the opening of sarcolemmal $K_{ATP}$ channel in rat hearts with the minimal vasorelaxant effects.