Cineradiographic studies on the cardiovascular system in snakes form the basis of this report. The method provides anatomical informations at the functional level and differs greatly from classical anatomical studies. The physiological interpretations involve timing of the various heart phases and the active role of the right atrium during ventricular filling. The snake ventricle has an incomplete septum and the admixture problem is discussed, as are also some considerations regarding the pulmonary circulation and the reserve mechanism of the snake heart.