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The primary mechanism of action of the smooth muscle of the veins upon their contained blood is one of imparting pressure to the blood by a surrounding force. This mode of action is to be distinguished from that of the smooth muscle of the arterioles, which is one of maintenance of arterial pressure through resistance to the outflow of blood. The function of the arterioles as resistive elements in the circulation is evaluated by the relationship between the volume of the flow of blood through the arterioles per unit period of time and the change in pressure across the arterioles. The function of the veins as containers of the blood volume is best evaluated by the relationship between a change of the volume of the veins and the associated change of the pressure of the veins.It is the purpose of this paper to describe a new method for the simultaneous study of the functions of the arterioles and the veins of the extremities of man. In addition, the effects of several stimuli upon veins and arterioles of the forearm are also described. The stimuli employed were the reduction of the temperature of the room, the reduction of the temperature surrounding the forearm under study, and the congestion of the veins of the three extremities not being studied. METHODSA water plethysmograph made of lucite which had two chambers was used in all of the experiments (Figure 1). Each chamber was 8.5 cm. long and had separate sleeves which were thin, loose fitting rubber. The

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A method for measuring venous "tone" in the limb segment of man in vivo was used to study the problem of cardiac decompensation. Venoconstriction was present in patients with heart failure and it tended to remit as the patients improved.