A CO2 sensor using mass spectrometry is described. It responds linearly to O2 and CO2 with a time constant of 0.5 sec: it is not affected by pressure or flow in the physiological range: its temperature coefficient is 4 mm Hg PCO2 per degree C at 37 degrees C. 2. When this sensor and its through flow cuvette were placed in a common carotid artery-to-jugular vein loop in anaesthetized cats breathing spontaneously or being ventilated artificially, fluctuations of Pa, CO2 which had the same period as respiration were readily observed. 3. The amplitude of these fluctuations varied inversely with respiratory frequency being less than 1.5 mmHg Pa, CO2 in the range of normal respiratory frequencies in the cat, 25--35 min-1. The amplitude also varied with the mixed venous-to-end tidal CO2 difference which was altered either by giving the cat CO2 to inhale or infusing CO2 intravenously. 4. We have concluded that these fluctuations of Pa, CO2 are unlikely to provide a significant drive to ventilation at normal respiratory frequencies but they may provide a signal that gas exchange in the lung is less than optimal.
