It has been claimed that the most important factor in the causation of shock is diminution of CO2 within the blood, and that this diminution is a regular consequence of all influences resulting in shock. That CO2 possesses important physiological functions cannot be denied. An investigation therefore of the true significance of a diminution of its normal amount within the blood is important and bears a special relation to various methods of artificial respiration utilized in thoracic surgery. The present experiments were undertaken for the purpose of investigating the effects of acapnia and the relation of some factors concerned in its production to shock. In all of them dogs were used. The first series was performed for the purpose of studying the effect of variations in intrapulmonic air pressure upon the blood pressure. The thorax was opened laterally, a T-tube connected with a water manometer was tied in a small bronchus, and the heart enclosed in a Henderson cardiometer in series with a recording tam...