Injection of radio-opaque material into specific areas of the human heart has become increasingly popular with the increasing use of diagnostic angiocardiography. An investigation of the effect of high pressure (600 lbs./sq. inch) injection of contrast media was undertaken in a series of 48 experiments on 38 mongreal dogs. Subendocardial hemorrhages and intracardiac thromboses were found when catheters remained in the heart for over 45 minutes. No such lesions could be attributed to high pressure injection of ordinarily used contrast agents. Disturbed pressure relationships were frequent with injection into the heart, superior vena cava, and pulmonary artery. Such electrical, contractile and pressure disturbances were not noted with injection into the inferior cava or aorta.