The determination of the circulation time from the ventricle to the face using fluorescein and ultraviolet light eliminates all of the variable factors from the venous side of the circulation and gives evidence specifically of the presence or absence of an intraventricular right to left shunt. The most important clinical application is the differentiation of tetralogy of Fallot from pulmonary valvular stenosis with an intact ventricular septum, the first requiring a shunting type of operation and the second a pulmonary valvulotomy.