In order to study ventilatory mechanics in shock, dogs were bled arterially into a reservoir, the height of which was regulated to keep the mean blood pressure of the animal at approximately 30 mm Hg. When the animal "took up" 40% of his maximal shed volume of blood (2-3 hr), the remainder of the blood was reinfused and the animal assumed to be in irreversible shock. Studies throughout the stages of hypovolemic and irreversible shock revealed a significant rise in lung compliance and a fall in combined viscous and air-flow resistance initially if the animal''s lungs were carefully inflated prior to each study. As shock continued, there was a tendency for the lung compliance and resistance to air flow to return in the direction of the control values.