The common practice of using alkalotic cardioplegic solutions is not supported by experimental evidence. The present study was conducted (1) to assess the effects of varying the pH (7.00, 7.40, and 7.70 at 20°C) of a glutamate-containing cardioplegic solution on intracellular pH, high-energy phosphate content, and postarrest functional recovery and (2) to compare the effects of various buffers (glutamate, bicarbonate, TRIS, and histidine) at a given pH (7.00 and 7.40). Isolated perfused rat hearts were subjected to 2 hours of cardioplegic arrest at 15° C followed by 30 minutes of reperfusion. Intracellular pH and high-energy phosphate content were measured at 4 minute intervals by phosphorus 31 nuclear magnetic resonance spectroscopy. These data were correlated with postischemic recovery of function. There was no significant difference between the intracellular pH values recorded at the end of arrest in the three glutamate-containing groups. However, the acidotic solution (pH 7.00) resulted in better preservation than the alkalotic solution (pH 7.70), as evidenced by (1) a higher creatine phosphate content at the end of arrest (61% ± 9% of control values versus 30% ± 9% [mean ± standard error of the mean], p < 0.05), (2) a higher adenosine triphosphate content at the end of reperfusion (102% ± 5% versus 82% ± 6%, p < 0.05), and (3) a faster recovery of aortic flow (at 3 minutes of reperfusion, 91% ± 11% versus 51% ± 11%, p < 0.05). Subsequent comparison of buffers showed that bicarbonate, TRIS, and histidine were equally effective in maintaining intracellular pH close to control values during arrest. Conversely, the use of glutamate resulted in a more pronounced fall in intracellular pH, which correlated with a better preservation of adenosine triphosphate and a better functional recovery than in the other groups. Overall, the greatest extent of preservation was provided by the pH 7.00 glutamate-containing cardioplegic solution. We conclude that additional protection can be conferred to the cold, chemically arrested heart by combining (1) mild intracellular acidosis, which lowers metabolic needs during arrest, most likely through a limitation of calcium overload, and (2) provision of glutamate, which may act as a substrate for anaerobic energy production while allowing intracellular pH to be kept within the appropriate range.