Abstract: A method and apparatus for obtaining non-invasive cardio-pulmonary measurements, without voluntary patient cooperation, by means of an inhalation indicator dilution rebreathing device having a valving system and test gas chamber connected to a volume ventilator, the valving system being operable to connect the ventilator directly to the patient for normal ventilation or to effect inhalation and exhalation of the test gas during the measurement cycle, the test gas being monitored and analyzed by a test instrument such as a mass spectrometer to ascertain the cardio-pulmonary parameters.