To assess the value of echocardiography for detection of the flow-dependent epicardial coronary vasodilation, the changes in internal diameter of the left anterior descending coronary arteries (LAD) induced by reactive hyperemia were studied by echocardiography in 12 health anesthetized open-chest dogs. Reactive hyperemia was induced by brief occlusion of the left anterior descending coronary artery for 30 s followed by rapid release. The two- dimensional images of the left anterior descending coronary artery before and after reactive hyperemia with and without intracoronary infusion of NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase(NOS) were investigated. The internal diameter of LAD was measured and its percent change induced by reactive hyperemia was calculated. Our results showed that the internal diameter of LAD was 2.23±0.19 mm before intracoronary infusion of L-NAME (baseline). The internal diameter of LAD significantly increased to 2.52±0.24 mm (P＜0.01) after reactive hyperemia at baseline, and the percent change in internal diameter of LAD was (13. 10±3.59) %. The internal diameter of LAD before and after reactive hyperemia under the condition of intracoronary infusion of L-NAME was not different from that before reactive hyperemia at baseline. The percent change in internal diameter of LAD was (1.07±2.97) %, and it was significantly lower than that at baseline (P＜0.001). We are led to conclude that the change in internal diameter of LAD responding to reactive hyperemia was detected sensitively by echocardiography, and this change was associated with endothelium-derived nitric oxide.