The effect of verapamil (Isoptin) on coronary hemodynamics was studied in 18 patients with coronary heart disease. The continuous infusion thermodilution technique was used to measure cardiac venous blood flow. At rest, verapamil produced a 13% decrease in coronary arteriolar resistance, a 5% decrease in coronary arteriovenous oxygen difference and a 19% increase in left ventricular efficiency index. No significant effect was noted on myocardial blood flow, oxygen consumption or oxygen consumption per unit of left ventricular pressure work. No change occurred in any of the measured parameters during a submaximal stress test after drug administration. The findings are discussed in relation to the general hemodynamic effects of the drug. It is concluded that verapamil is a mild coronary vasodilator and that myocardial oxygen consumption, in relation to performed left ventricular work, is unaltered by the drug up to a near-maximal work-load.