A small electromagnetic flowprobe was chronically implanted in circumflex branch of left coronary artery of mongrel dogs. Diltiazem, 100μg/Kg, was injected in a basilic vein of the unanesthetized dogs. The changes in phasic coronary flow as well as hemodynamics including aortic pressure (AP), heart rate (HR), and myocardial oxygen extraction ratio (OER) were in estigated. The increasing rates of total coronary flow (TCF) 5, 10, and 20min after the injection of the drug were 90%, 65%, and 40% respectively, Total coronary vascular resistence (TCVR) was inversly proportional to TCF, resulting in a significant decrease after injection. Five min after the drug administration, OER markedly decreased to the level of -45% of control value. The changes in aortic pressure (AP) and heart rate (HR) were unremarkable.In clinical investigation, coronary atrery narrowings (spasms) observed by cine coronary arteriography in a patient with a variant form of angina pectoris were completely dilated after intravenous injection of Diltiazem.In conclusion, Diltiazem exerted a powerful and long-acting vasodilator action on coronary vasculature. Although AP and HR showed unremarkable changes after the injection of the drug, OER decreased significantly. Results obtained suggest that Diltiazem can reduce myocardial oxyge consumption and OER and that the decrease in OER is attributed to not only the peripheral hemodynamic effect of the drug but also probable consequence of the direct action on the myocardium.