Abstract: Atmospheric air is introduced into reference and sample chambers, whereupon alternating circulation of the gases in the two chambers through a carbon dioxide sensor, an oxygen sensor, a pressure sensor and a pressure regulator occurs before the gases are returned to their respective chambers. The calculation of rate of production or consumption of carbon dioxide and oxygen by a tissue or other sample in the sample chamber is controlled by a microprocessor which receives signals from the carbon dioxide, oxygen and pressure sensors and controls the circulation of the gases. Sensor drift is compensated for in the calculation through the use of multiple reference chamber readings and through volume determinations for the reference and sample chambers and the sensors.