A system for on-line measurement of respiratory gas exchange in patients undergoing artificial ventilation is described. Fractional concentrations were measured by a mass spectrometer and expired flow by a pneumotachometer; signals processing was carried out by a microcomputer. The accuracy of the measurement of the burning methanol RQ was within 1.6% at FIO2 lower or equal to 0.4. When compared to a reference method (the Douglas bag method), correlation was found to be excellent. The major problem in measuring gas exchange in a ventilated patient is that physiological signals are contaminated by artefacts which may lead to erroneous computations. An algorithm was developed in order to identify and disregard artefacted periods or ventilatory unsteady state in the patients. The technique used for data processing gave reliable continuous measurements of respiratory gas exchange for periods up to 24 h, without interfering with the nursing of the ventilated patients.