Closure of the caval orifices of the right atrium and dynamics of the venae cavae have been studied by an analysis of high-speed cineangiocardiographic records. The influence of respirogenic and cardiogenic factors on venae cavae dynamics was demonstrated. Findings indicated that mechanical and hemodynamic factors interact in directing the atrial blood forward and in preventing its reflux into the venae cavae. Relative narrowing of the caval orifices was demonstrated during atrial systole. The degree of narrowing, however, was insufficient to produce a mechanical closure and thus to prevent caval refluxes when the pressure gradients were altered. The findings are illustrated and discussed.