Abstract: The purpose of this study was to define the effects of nitroglycerin on coronary circulation in ischemic region. In 18 open chest dogs, a critical stenosis of the left anterior descending coronary artery was produced by an external micrometer constrictor. Intracoronary infusion of nitroglycerin 0.5μg/kg, min^-1 increased coronary blood flow (CBF) and decreased distal coronary pressure (DCP), total coronary artery resistance (R_T), small coronary artery resistance (Rs); but did not affect mean aortic pressure, heart rate and large coronary artery resistance (R_L). The whole blood viscosity in coronary vein was reduced. Intracoronary infusion of nitroglycerin 1μg/kg.min^-1 CBF increased and R_T, R_L, R_S, DCP decreased during the first 5min. Then the CBFdecreased, Rr and R_L increased. These were associated with a decrease in DCP and an increase in whole blood viscosity. A passive narrowing of the stenotic area was observed.These results indicate that nitroglycerin has two effects on coronary circulation, it can increase CBF and decrease coronary artery resistance in relieving myocardial ischemia; it can also decrease CBF and cause passive narrowing of coronary artery in worsing myocardial ischemia. The beneficial or harmful effect of nitroglycerin may depend on its doses and DCP.