The haemodynamic effects of a new anti-anginal drug, dilitazem HCl, were studied in 20 patients with angina pectoris. The 131I-dilution method was used. In two patients, one with atrial septal defect and the other with pulmonic regurgitation, heart catheterization was carried out. The drug exerted significant decrease in stroke volume, cardiac output, cardiac work, stroke work and cardiac output/circulating blood volume ratio. The patients who underwent direct cardiac catheterization disclosed no changes in Vmax and ejection fraction, but there were decreases in stroke volume. The decrease in cardiac work and stroke work after after diltiazem Hcl therapy was apparently due to diminution of venous return and prolongation of systemic circulation time. It is postulated that such a decrease leads to inhibition of myocardial oxygen consumption, which in turn has beneficial effects on angina pectoris.