Serial determinations of the cardiac output, blood gas, and electrolyte values have revealed three basic patterns of circulatory response in 42 patients whose respiratory tidal gas exchange was assumed by a volume displacement respirator for periods up to 12 days: 1. In 15 patients with severe chest and pulmonary injuries, the cardiac indices increased from very low levels (average 1.7±0.3 L/m 2 /min) to 2.8±0.7 L/m 2 /min as profound hypoxia and acidosis were corrected. 2. Eight patients with severe bronchial obstruction or respiratory insufficiency (emphysema or pulmonary fibrosis accompanied by pneumonitis) responded by a decrease of the cardiac output from an average of 3.3 L/m 2 /min to 2.4±0.7 L/m 2 /min. Usually, in a state of combined respiratory and metabolic acidosis (pCO 2 81 mm Hg, pO 2 47 mm Hg), these patients became alkalotic as the arterial pCO 2 approached 40 mm Hg. 3. Nine patients with low cardiac outputs (average 1.4±0.4 L/m 2 /min) following cardiac operations were markedly improved by use of a respirator. Although there was little change in the cardiac output (average 1.4±0.3 L/m 2 /min) for one or two days, metabolic acidosis was dramatically reduced (–7.4 to –1.3 mEq/L). A circulatory output previously inadequate for body perfusion became adequate when the work of respiration was removed. Ten patients with combined respiratory and cardiac dysfunction exhibited mixed circulatory reactions to the use of a respirator. From these data, it is concluded that mechanical ventilation can improve an abnormal metabolic status due to cardiac or pulmonary dysfunction by increasing blood gas exchange. When a normal cardiovascular system is present and capable of responding, cardiac output rises to a value sufficient to satisfy the total body requirements for circulation. By decreasing the perfusion requirements of the respiratory muscles, the total demand for circulation is reduced, and perfusion to other parts of the body is improved in patients incapable of elevating the cardiac output.