We measured coronary sinus blood flow by continuous thermodilution technique and aortic pressure after administration of diltiazem to 23 patients with coronary artery disease. In one group of patients (n = 12) the drug was infused at a rate of 0.15 mg/kg during 2 min followed by an infusion of 0.05 mg/kg during 8 min. Heart rate was unchanged except at 5 min when it decreased slightly. Aortic pressure was significantly (p less than 0.01) decreased, while coronary sinus flow increased slightly and transiently. A second group of patients (n = 5) received an intracoronary injection of 0.15 mg/kg of diltiazem into the left coronary artery. In a third group of patients (n = 7) 0.05 mg/kg of diltiazem was injected into the left coronary artery. In both these two groups the drug induced a marked increase of coronary sinus flow and a decrease of aortic pressure, while myocardial oxygen consumption was unchanged. This effect was dose related, since the rise in coronary flow was 47% with an injection of 0.15 mg/kg but only 23% with a dose of 0.05 mg-kg. These changes were short-lasting with values returning to normal within 10 min after the injection. We conclude that diltiazem is a potent dilator of coronary arteries.