Intravenous injection of the selective mu-opiate receptor agonist DAMGO (0.1 mg/kg, 15 min before isolation of the heart) improved resistance of isolated perfused rat heart to ischemia (45 min) and reperfusion (60 min) damages. In vivo administration of DAMGO prevented reperfusion-induced damages to cardiomyocytes and decreased the content of conjugated dienes in the myocardium during ischemia-reperfusion in vitro. Furthermore, stimulation of mu-opiate receptors promoted recovery of myocardial contractility during reoxygenation, but had no effect on heart resistance to free radical-induced damages during perfusion of isolated heart with a solution containing Fe2+ and ascorbic acid.