We describe a single flow transducer breath-by-breath gas exchange measurement system suitable for use during general anaesthesia. The system uses a Fleisch No. 2 pneumotachograph, a mass spectrometer and a microcomputer to give real-time continuous measurements. Correction for apparent gas exchange attributable to changes in gas stored in the lung (functional residual capacity) is available. The correction assumes no gas exchange of either nitrogen or argon for air-breathing subjects or argon only during anaesthesia, while the inspired concentrations are maintained at atmospheric values. The method has been tested against Douglas bag measurement and compared with results from conventional systems used by other authors. The system measurements show broad agreement with Douglas bag measurements, although the limits of agreement are wide for air-breathing volunteers. The system between-breath variation was typical of breath-by-breath methods in other areas of medical research.