The authors address the following problems: How to turn a mass spectrometer, or a set of individual gas sensors, into a real and useful medical instrument? In other words, how to transform the instantaneous gas composition signals into meaningful physiological variables? The parameters that can be computed breath by breath from the real time processing of gas concentration signals, combined with flow and pressure signals at the mouth are first described. Particularly, we point out the theoretical and practical importance of alveolo-capillary gas exchange parameters, as opposed to gas exchange parameters estimated at the mouth level: A-c exchange parameters are a more sensitive and more specific indicator of any physiological change and they are less sensitive to breath by breath fluctuations of ventilation. We discuss the clinical usefulness of breath by breath computations, as a more sensitive way to monitor the patient as well as the anesthesia circuit, and to generate all the information required for on line analysis of functional tests. We describe a system for the real time processing of the respiratory signals. Based on three microprocessors it takes over the calibration, the offset correction of each signal ... It also corrects for the dynamics of each sensor and resynchronizes all the signals. It computes breath by breath more than 50 physiological variables that can be either recorded analogically, either printed, either acquired by a general monitoring system of the patient, which then combines respiratory data with other physiological. therapeutical and medical data from the patient.