This project involves the development of a system to measure blood PO2 and PCO2 in extracorporeal circuits for 72 hour periods without blood loss. Picomole amounts of oxygen and carbon dioxide are extracted from the circulation with a special flow through probe or catheter probe. Oxygen is analyzed electrometically and carbon dioxide is measured by calorimetry of the reaction with carbon dioxide and lithium hydroxide. Tests have shown the system to be stable within five percent over 72 hour periods.