This study was designed to investigate the protective effect of chlorpromazine against the reperfusion injury of myocardium after high potassium cardioplegic arrest. Langendorff`s preparations of rat heart were infused with high potassium cardioplegic solution[St. Thomas Hospital Solution] at 25oC. Chlorpromazine [10-7Ｍ] increased the recovery of myocardial contractility[dp/dt], left ventricular pressure[LVP], and coronary flow rate of the reperfused heart. Both in control and experimental groups, the restoration of myocardial activity could not reach to the level of preplegic control. These results suggest that the etiologic factors of the reperfusion injury include the influence of high potassium cardioplegic solution and/or reperfusion itself, and that chlorpromazine protects myocardium from the reperfusion injury.