Summary Reperfusion of ischemic myocardium may aggravate the ischemic state of injury and thus augment infarct size (reperfusion injury). The aim of this study was to reduce infarct size by an intervention at the time of reperfusion that acts only on a reperfusion-specific pathomechanism. It was investigated whether SIN-IC, a metabolite of molsidomine, can protect against reperfusion injury in canine hearts in vivo. Ten anesthetized open chest dogs underwent 1 h of left anterior descendent artery (LAD) occlusion and were randomly assigned to receive either intracoronary SIN-IC or vehicle infusion as a placebo during the first hour of reperfusion. The infusion was adjusted to LAD flow to achieve a regional blood concentration of 5 × 10 3M. Infarct size was assessed by triphenyltetrazolium staining after 6 h of reperfusion. Left ventricular pressure (LVP) was similar in both groups (SIN-IC: 101 ± 6. placebo: 89 ± 6 mm Hg. mean ± SEM, n = 5) at the beginning of the experiment and did not change significantly thereafter from baseline values in both groups. During SIN-IC infusion, the LAD flow was increased (SIN-IC: 195 ± 38, control: 86 ± 17 ml/min/100 g at 30 min of reperfusion. p < 0.05). while systemic hemodynamics remained unaltered. A reduction in infarct size (percent of area at risk) was seen in the SIN-IC group (11.4 ± 2.8%) compared with the placebo group (24.4 ± 3.99%. p < 0.05). Infusion of papaverine (5 × 10-5 M) following an identical protocol caused a similar vasodilation as SIN-IC, but did not reduce infarct size in five additional dog experiments. Infusion of SIN-IC (2.5 × 10-3 −10 -2 M) in normal myocardium in one additional dog showed a dose-dependent increase in LAD flow to 270%, which was paralleled by a depression of regional wall thickening (33%). The results demonstrate that necrosis in the reperfused myocardium can be greatly reduced by therapeutic intervention at the onset of reperfusion. Intracoronary SIN-IC can be used for such an intervention. It has a pronounced local protective effect, without altering systemic hemodynamics. The mechanism of protection does not seem to be related to the coronary vasodilatory effect of SIN-IC.