Oxygen uptake (VO2) and carbon dioxide output (VCO2) were measured continuously in four children during cardiopulmonary bypass, using the variation in concentration of a tracer gas, argon, as a measure of flow, and a mass spectrometer to monitor gas concentrations. Normal values for the ratio of VCO2:VO2 (= 0.8) indicated that gas exchange equilibrium had been established following sudden changes in body temperature. During rapid cooling, 10% carbon dioxide is necessary to ensure carbon dioxide homeostasis within the body.