Background: Nitric oxide (NO) strongly dilates the resistance arterioles and plays an important role in coronary blood flow through the microcirculation. Contrary to nitropruside, nitrogylcerin has to be metabolized by the vascular wall. So nitroglycerin expected not to play in effect role in microcirculation. We aimed to reveal the responses of coronary artery blood flow to nitropruside and nitroglycerin. Methods: We enrolled 59 consecutive subjects and divided them into noncardiac chest pain (NCC) and microvascular angina (MA). Exclusion criteria were myocardial infaction, congestive heart failure, unstable angina,vasospastic angina patients. Baseline coronary angiography was done in RAO caudal view of LAD. We injected 15µg of nitropruside to LAD through the diagnostic catheter and angiography (15 frames/sec) was done in 30 seconds. After 3 minutes, angiography was done for intermission, after then we injected 150µg nitroglycerin to LAD and angiography was done. TIMI frame count was calculated in LAD respectively.Results: TIMI counts decreased significantly after injection of nitropruside. And TIMI count restored and increased significantly at intermission. But for nitroglycerin, TIMI count decreased significant in MA group only. Conclusions: We observed that direct infustion of nitropruside decreased TIMI counts in both group and increased in intermission. We can hypothesis that direct NO donation can increase counteracting vasoconstrictor so TIMI count increased at intermission, and make different resonse to nitroglycerin.