The study of the venous pressure by the direct method of Moritz and von Tabora (1) has been of great value in the diagnosis and physiological understanding of heart failure and a number of other conditions. Provided a certain technique in measurement, as outlined by Lyons, Kennedy, and Burwell (2), is observed, it is commonly assumed that the arm venous pressure is a function of the atrial pressure, since the vein is a tube connecting the antecubital space with the heart. It is our purpose to show that the peculiar nature of this tube, in that it is easily collapsible, places important restrictions on this interpretation. The critical experiment in this regard is the classical phenomenon, studied by many observers, of the behavior of the venous pressure as the arm is raised (Fig. 7). Whenthe pressure is referred to the heart level, the pressure is constant until the arm reaches a certain point, and then rises equally with further elevation. When the pressure is referred to the vein level, the pressure falls until this same point is reached, and then remains constant. Observation indicates that below this point, the vein is distended, and at and above this point, the vein is collapsed. It is with the full interpretation of this phenomenon that this paper deals. It is our purpose to show that when the vein is distended, it does measure the central venous pressure, and that when it is collapsed, it is completely independent of central influences and measures only the tissue pressure about the vein at the point of measurement. This will be done by showing, first, that a collapsible tube behaves in this way; second, that an isolated segment of vein has the properties of a collapsible tube; and third, that the vein behaves in vivo just as it does in vitro. MATERIALS AND METHODS