pH and blood gases were measured in simultaneous samples of arterial blood from the radial artery and mixed venous blood from the pulmonary artery using an ABL300 and OSM3 (Radiometer A/S, Denmark). Cardiac output was measured by thermodilution. The patients were suffering from chronic obstructive pulmonary disease or adult respiratory distress syndrome. The data indicate that patients respond to a decreased arterial oxygen availability by allowing the mixed venous pO2 to fall rather than by increasing the cardiac output to maintain a normal mixed venous pO2. In other words, the arterial oxygen extraction tension and the oxygen compensation factor were both highly correlated to the mixed venous pO2 but unrelated to the cardiac index. For this reason the arterial oxygen extraction tension appears to be a more relevant parameter of the overall arterial oxygen availability than the oxygen compensation factor. Comparison of the arterial and mixed venous data confirms the accuracy of the Oxygen Status Algorithm for calculating the various oxygen parameters, including the p50, the estimated 2,3-diphosphoglycerate concentration, and the estimated physiological shunt, on the basis of a single arterial blood sample.