The technic employed in catheterization of the left heart in man is (lescribe(l. A catheter is introduced into the left ulnar artery and passed through the brachial, axillary and subclavian arteries into the arch of the aorta. With the tip of the catheter at the root of the aorta, we have succeeded in entering the left ventricle only in patients with free aortic insufficiency due to syphilis. Failure to pass the aortic valves in normal subjects is discussed. S INCE the earlier work of Cournand andRanges,' Stead and his associates,2 MIcMichael and Sharpey-Schafer,3 and others, catheterization of the right ventricle and pulmonary artery in man has become a standard procedure which not only supplies valuable data in the accurate diagnosis of congenitally malformed hearts, but which is also applicable to the study of a variety of problems in the cardiovascular field. A considerable experience with catheterization of the right side of the heart prompted us to try the procedure on the left side.Catheterization of the left side of the heart presents obvious problems not encountered on the right side. The catheter must move retrograde against arterial pressure while arterial vasospasm may be so marked that the catheter cannot be passed forward. After reaching the aortic root it must be moved through the orifice into the left ventricle against the blood column and in that short ejection interval (0.22 second) during which the aortic valves are open.The exact position of normal aortic leaflets ,during ventricular systole in the intact heart is not known. If the pressure difference in the left ventricle and aorta were the only factors concerned the valves would lie snugly against the intima of the aorta, but other subsidiary forces may actually move the valve toward a position of closure during the ejection phase.As Wiggers4 has stated, "Among these ac-