The hemodynamic changes resulting from injection of radiopaque material into the left heart in a series of patients undergong cineangiographic studies have been reported. The hypertonicity of radiopaque materials appears to be responsible for much of the observed reaction. The mechanism whereby hypertonic solutions produce the observed physiologic changes remains unknown.Changes observed in patients could be reproduced in experimental animals. The combined experimental and clinical data show that left atrial pressure increases, left atrial pulse contour alters, stroke output increases, heart rate is much unchanged, peripheral artery pressure falls, hematocrit level falls, and myocardial contractile force decreases mildly and transiently.The difference between the physiologic effects of injecting hypertonic media into the right and left sides of the circulation is discussed.Because the pressure changes are easy to monitor and parallel the other features of the hemodynamic reaction, it is good to wait until pressures have returned to the pre-angiographic level before proceeding with the injection of more radiopaque material. This usually requires 15 minutes.