Intervention: The STEMI patient with 1 significant occlusion undergoing PCI less than 6 hours from onset of symptoms will be randomized to intracoronary injection of either melatonin or placebo(saline). The melatonin or placebo will be given as a bolus of 10 ml in relation to PCI, as soon as the occlusion is removed. Immediately after the bolus is given, a solution of 490 ml of either placebo (saline) or 0,1 mg/ml melatonin (49 mg melatonin)will be given intravenously over 6 hours (80 ml/hour).