Pulmonary embolism secondary to venous thrombosis, especially in the deep veins of the legs, is a disorder that has been feared by physicians for centuries. Since Valsalva's1 classic experiment in 1740 in which he described the effects on man of holding the breath in inspiration and making an expiratory effort with the glottis closed, there have been numerous studies. on venous pressure. However, it was not until the method of Moritz and Tabora2 was introduced in 1910' that comparable measurements of the venous pressure were recorded. They and subsequent investigators noted a great increase in venous pressure in subjects performing the Valsalva experiment but on review of this literature we find that their observations have been chiefly on the veins of the arm: the physiologist Mean3 did observe a great increase in the volume of the peripheral veins in both the arms and the legs.