A simple device was developed which can accurately measure effective changes in total vascular capacity as viewed from the site of the heart. The device consists of a funnel and a suction bottle. It is connected via a single large-bore cannula to the right atrium of an open-chest animal without opening the closed loop of the circulation. It serves to clamp the steady-state level of mean central venous pressure precisely at any desired value and to measure the volume of any displacement of blood out of or into the total vascular bed caused by changes in total vascular capacity. The device was tested in mock and animal experiments and proved to function accurately, except during a short transient phase of less than 1 min following an intervention on the total vascular capacity. Errors in clamp pressure in the steady state, which are theoretically zero, were found to be less than 0.2 mm Hg, and were practically insignificant. Errors in the volume measurement were less than 5 ml.