The importance of nitric oxide (NO) in coronary blood flow (CBF) regulation was examined in anesthetized pigs. NO synthesis was inhibited by intracoronary infusion of NG-monomethyl-L-arginine (L-NMMA) or NG-nitro-L-arginine (L-NNA). L-NMMA (30 mumol/min) reduced CBF (Doppler flowmetry) by 16.3% (13.1-20.2%; P < 0.001) and L-NNA (30 mumol/min) by 16.1% (13.9-18.9%; P < 0.001). During NO blockade, myocardial oxygen consumption was unaltered as an increase in oxygen extraction occurred due to a reduced partial pressure of oxygen and oxygen saturation in blood from the anterior interventricular vein. L-Arginine completely reestablished CBF after giving L-NMMA, but not after giving L-NNA. L-NNA reduced the coronary flow response to ADP by 66-83%, whereas the selected dose of L-NMMA did not affect it. The flow response to adenosine was not affected by either L-NMMA or L-NNA. L-NNA reduced reactive hyperemia after occluding the left anterior descending coronary artery for 10 and 30 s but not for 120 s. Our data show that NO produced in the coronary endothelium plays an important role in CBF regulation in vivo, accounting for approximately 16% of CBF and a major part of the flow response to ADP. NO also contributes to reactive hyperemia after brief, but not longer, ischemic periods.