Abstract: A method and apparatus for measuring or monitoring pCO.sub.2, and optionally pO.sub.2, on a real-time basis, in the stomach or the intestine using air tonometry. The intraluminal measurements are made using a catheter having a CO.sub.2 sensor, and optionally a pO.sub.2 sensor, at its tip for in situ measurements, or by circulating a fixed amount of air through the catheter with measurement occurring at the proximal end of the catheter. Instrumentation supporting the sensor may be adapted for location within a patient or external to a patient.