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The development of an antiomony electrode (Sb) and a pH meter as a substitute for a glass electrode(G) to measure the intramyocardial pH is reported, and the results of their clinical application. The determination of the pH of CPD (citrate-phosphate-dextrose) blood by Sb showed small differences when compared to G (less than 0.1). Sb was also temperature sensitive. The temperature coefficient of Sb was determined in buffers by varying their temperatures, and an antimony-thermocouple electrode and a temperature-compensated pH meter were subsequently constructed. In CPD blood of varying temperatures Sb showed similar tendencies to G. The results of the application of the new apparatus to 28 patients were statistically no different from those obtained previously by G on 22 patients, either in baseline pH or its fall following aortic cross-clamping. It is concluded that although Sb is not as accurate as G, it is a reasonable alternative for the determination of myocardial acidosis during open-heart surgery.