Echinochrom, a new antioxidant of the polyhydroxynaphthaquinone class, was tested for its cardioprotective activity in a model of occlusive reperfusion myocardial infarction (90-min occlusion and 4-hour reperfusion) in the acute experiments with open-chest dogs. The bolus intravenous injection of echinochrom in a dose of 1 mg/kg 5 min before reperfusion caused a significant (over 40%) reduction in the size of a necrotic focus. A supplementary administration of echinochrom 5 min after the onset of ischemia failed to contribute to a significant enhancement of its protective effect, suggesting that there is no substantial effect of the agent on ischemic lesion. The efficacy of echinochrom given after prolonged ischemia, low effective doses, and no adverse effects create prerequisites for using the drug in the clinical setting.