Abstract: In the portable instrument an intragastric probe, with a combined pH-electrode at its tip, measures pH values over a long time period and stores, calculates, evaluates, indicates (at any selected time) and prints out the data. At any particular time, both the actual pH value at that moment and all stored data up to that moment are immediately available to the physician. The small size of the apparatus does not interfere with the normal activity of the patient even over a long time period. The apparatus allows the individual diurnal rhythm of acid secretion to be observed, including the effects thereon of food (and dietary changes, smoking, alcohol, coffee, and drugs, as they occur.