Direct measurements of the venous pressure in dogs in the horizontal and 2 vertical (head-down, head-up) positions showed that the heart is not the point from which the hydrostatic factor in venous pressure is measured. In the dead dog the venous system acts as an unbroken column from head to tail; the reference point from which hydrostatic pressure is measured was 82 mm. caudal to the heart in the vertical position in a dog of ca. 500 mm. spinal length. In the living dog the venous system is broken at the heart, giving 2 columns with a reference point ca. 121 mm. from the heart in the tail section and another reference point 38 mm. from the heart in the head section.