Electro-magnetic flow studies were performed on the ascending aorta, superior vena cava, femoral artery and perfusion circuit during cardiac surgical procedures on patients. The safety of the study technic was established. Pilot observations to date include: Normal ascending aorta phasic flow pattern. Flow patterns with abnormal cardiac rhythms. Flow tracings in aortic valvular insufficiency with indications of marked improvement after valvuloplasty or valvular replacement. Evidence that about 40% of cardiac output is represented by superior vena caval inflow. Indication that peripheral vasomotor tone may be lost during perfusion, with prompt return on cessation of perfusion. A possible aid in diagnosis of sub-valvular aortic stenosis.