Effects of the new Ca++ antagonist nifedipine (Adalat) on coronary haemodynamics were studied in 8 patients with documented ischaemic heart disease. The continuous infusion thermodilution technique was used to measure cardiac venous blood flow. Sublingual application of 10 mg nifedipine caused a significant increase (16%) in myocardial blood flow and a decrease (18%) in coronary arteriolar resistance at rest, but not during a submaximal atrial pacing test. There was no change in coronary arteriovenous oxygen difference, myocardial oxygen consumption, oxygen consumption per unit of heart rate blood pressure index or left ventricular efficiency index. The effects on the coronary haemodynamics are discussed in relation to the simultaneous changes in general haemodynamics. Systolic aortic pressure was slightly reduced, significantly only at rest, while peripheral vascular resistance decreased and cardiac output increased also during atrial pacing. No change in free fatty acid metabolism was observed. It is concluded that nifedipine is a mild coronary vasodilator. No effect was observed on myocardial oxygen demand. The oxygen cost of left ventricular work was unchanged by the drug both at rest and during the submaximal stress test.