The effect of profound hypercapnia on acute hypoxic respiratory failure is evaluated. Eight dogs were subjected to oleic acid-induced acute respiratory failure. Four dogs were ventilated normally, and four dogs were made hypercapneic by rebreathing exhaled CO2. In the hypercapneic animals, heart rate and alveolar-arterial oxygen difference were significantly lower than in normocapneic animals, while mixed venous O2 cardiac index, oxygen delivery index, stroke volume index, and left ventricular stroke work were significantly higher. Mean arterial pressure was maintained at preinjury levels. Pulmonary and systemic vascular resistance increased in both experimental groups. There was no significant difference between groups for gravimetric determination of lung water. Cardiopulmonary performance in acute respiratory failure is improved with hypercapnia. This may be related to CO2-induced catecholamine release.