In evaluating patients for cardiac surgery the usefulness of hemodynamic measurements in the left atrium and ventricle has stimulated the development of several catheterization techniques. This report describes in detail the use of percutaneous methods for inserting the transseptal needle and the retrograde aortic catheter. Retrograde catheterization of the aortic valve, in the absence of significant stenosis, has been accomplished in 176 of 182 patients. Precautions that should prevent complications from the transseptal technique are described. The desirability of simultaneous measurements from the left atrium and ventricle for evaluation of the mitral valve is emphasized.