The single breath test for carbon dioxide (SBT-CO2) is the plot of expired FCO2 or CO2% against expired volume. It can be monitored during anaesthesia and in the intensive care unit with modest additions to generally available equipment. This paper describes some aspects of a computer program for presenting SBT-CO2 during controlled ventilation, in particular, the corrections to the primary data necessary for scientific accuracy. Examples are given of how the use of SBT-CO2 has increased our understanding of factors which influence the arterial-end-tidal PCO2 difference (PaCO2-PE,CO2). PaCO2-PE, CO2 is, in a given individual, usually dependent on tidal volume and frequency. Changes in lung volume and manoeuvres such as opening the pleura also affect gas exchange. Monitoring CO2 elimination gives a measure of metabolic rate if ventilation and pulmonary perfusion are maintained. This facilitates ventilatory therapy in situations where CO2 production is greatly increased, e.g. sepsis and tetanus. On the other hand, if metabolism and ventilation are unchanged, a reduction in CO2 elimination implies reduced pulmonary perfusion. This can be seen during increased right-left shunting, such as in surgery in patients with congenital heart disease.