31P NMR spectroscopy was used to study the time course of changes in the concentration of high-energy metabolites and intracellular pH in the dog myocardium during hypothermic ischaemia at 9 degrees C in Bretschneider (HTK-B) and St. Thomas' Hospital (StTH) cardioplegic solutions. It was found that ATP and phosphocreatine degrade slowlier in HTK-B than in StTH, with phosphocreatine depletion occurring within 7.9 +/- 1.4 h in HTK-B and within 6.2 +/- 1.4 h in StTH. The values are virtually identical with the time intervals at which ATP concentration falls below the critical level (60% of initial ATP concentration). In agreement with biochemical analysis, a higher concentration of phosphomonoesters was noted until the 180th minute of ischaemia in HTK-B, a finding suggesting more rapid glycogen degradation in HTK-B. Even though HTK-B contains a high concentration of histidine buffer, higher values of intracellular pH were found during ischaemia in StTH. The effect of extracellular concentration of sodium ions on intracellular pH is discussed.