In order to investigate the therapeutic effect of N-2-Mercaptopropionyl-glycine (MPG), a scavenger of hydroxyl radical, on postischemic myocardial dysfunction of 39 conscious unsedated dogs were undergone a 15 min occlusion of left anterior descending coronary artery followed by 48 hours of reperfusion. The treated dogs (n = 17) received an infusion of MPG (100 mg/kg.h) for 60 minutes (starting 15 minutes before occlusion) while the control dogs (n = 22) received normal saline. The result showed that there were no significant differences between the two groups in collateral blood flow determined by radioactive microsphere, occlusion vascular bed and hemodynamic variables. Systolic thickening fraction of ventricular wall (an index of regional myocardial function) was similar in both groups under baseline condition and during ischemia, whereas it (expressed as percent of baseline) was considerably greater in treated dogs at 2, 3, 4, 5, 6 hours of reperfusion. Much more significant recovery was observed in the dogs with collateral flow less than 10% in MPG group. The exponential regression analysis showed that the lower the collateral flow, the greater the recovery of function in treated dogs. It is concluded that MPG improves recovery of myocardial function after reversible regional ischemia, especially with lower collateral blood flow.