A small electromagnetic flowprobe was chronically implanted in circumflex branch of the left coronary artery of mongrel dogs. Diltiazem [a Ca antagonist] 100 .mu.g/kg, was injected in a basilic vein of the unanesthetized dogs. The changes in phasic coronary flow and hemodynamics including aortic pressure (AP), heart rate (HR) and myocardial O2 extraction ratio (OER) were investigated. The increasing rates of total coronary flow (TCF) 5, 10 and 20 min after the injection of the drug were 90, 65% and 40%, respectively. Total coronary vascular resistence (TCVR) was inversely proportional to TCF, causing a significant decrease after injection. At 5 min after drug administration, OER decreased to the level of -45% of control value. The changes in AP and HR were unremarkable. In clinical studies coronary artery narrowings (spasms) seen by coronary cinearteriography in a patient with a variant form of angina pectoris were completely dilated after i.v. injection of diltiazem. Diltiazem exerted a powerful and long-acting vasodilator action on coronary vasculature. AP and HR showed unremarkable changes after the injection of the drug but OER decreased significantly. Apparently, diltiazem can reduce myocardial O2 consumption and OER and the decrease in OER is attributed to the peripheral hemodynamic effect of the drug and probably to the direct action on the myocardium.