Objective To evaluate the myocardial protective effect of pretreatment with pinacidil, a potassium channel opener, on myocardium perfused intermittently with normothermic or hypothermic high potassium crystalloid cardioplegie solution during cardiopulmonary bypass ( CPB) . Methods Eighteen dogs of both sexes weighing 10-15 kg were intubated and mechanically ventilated after induction of anesthesia. Femoral artery and vein were cannulated for direct MAP and CVP monitoring. Swan-ganz catheter was placed via right internal jugular vein. The animals were randomly divided into 3 groups: group A (hypothermic CPB) in which heart was perfused with 4℃ St Thomas solution 10 ml?kg-1 in 2 min and 30 min later perfused again with half of the initial volume(5 ml? kg-1); in group B (normotherinic CPB) and group C (hypothermic CPB) heart was pretreated with oxygenated pinacidil solution (0.083 mg?kg-1 37℃) before cardioplegia. The heart was subjected to 60 min global ischemia followed by 30 min reperfusion. The hemodynamic changes were measured before aortic cross-clamping and 15 and 30 min after release of aortic cross-clamping. Myocardial tissue was obtained from apex of left ventricle for determination of myocardial adenine nucleotide content 5 min after CPB was initiated, 30 and 60 min after aortic cross-clamping and 20 min after reperfusion. Results During reperfusion the hemodynamic function and myocardial ATP content recovered significantly better in group C than those in group A and B ( P