The hypothesis tested is that shifts in pH, induced when a cardioplegic solution is oxygenated, can be detrimental. The object of this study is to evaluate the effect of the pH of the oxygenating cardioplegic solution on postischemic recovery in the isolated rat heart. Either 100% oxygen or 95% oxygen: 5% carbon dioxide was added to the cardioplegic solution[St. Thomas` Hospital No. 2] and determined postischemic recovery of isolated rat hearts after 2 hours and 3 hours of 20oC cardioplegic protected ischemia. Heart were arrested and reinfused every 30 minutes throughout the ischemic period with cardioplegic solution. When 100% oxygen was added, the pH of the cardioplegic solution increased from 7.8[no oxygen] to 8.5[100% oxygen] without any change in postischemic functional recovery. But when 95% oxygen ; 5% carbon dioxide was added, the pH of the cardioplegic solution reversely decreased to 6.84 in the 2-hour ischemic group and 6.73 in the 3-hour ischemic group, associated with improved postischemic functional recovery. After 2-hour ischemia, systolic pressure improved from 88.2$pm$3.7%[no oxygen] and 88.7$pm$3.8%[100% oxygen] to 96.6$pm$1.8%[95% oxygen : 5% carbon dioxide], p<0.05, aortic flow from 43.3$pm$3.1% and 38.4$pm$10.6% to 74.5$pm$5.0%, p<0.001, cardiac output from 55.5$pm$4.6% and 47.4%$pm$10.6% to 73.1$pm$4.6%, p<0.05, stroke volume from 62.7$pm$4.6% and 52.0$pm$10.1% to 77.2$pm$4.6%, p<0.05, and dP/dT from 59.3$pm$7.2% and 56.7$pm$7.6% to 78.9$pm$4.6%, p<0.05. The infused amount of the cardioplegic solution during 2-hour ischemic period was similar in three groups. After 3-hour ischemia, cardiac output improved from 17.0$pm$3.8%[no oxygen] to 45.9$pm$7.5%[95% oxygen: 5% carbon dioxide], p<0.05, and stroke volume from 21.0$pm$3.9%[no oxygen] to 50.1$pm$6.6%[95% oxygen: 5% carbon dioxide], p<0.01. In conclusion, the St. Thomas` Hospital No. 2 cardioplegic solution should be oxygenated but with 95% oxygen: 5% carbon dioxide and not 100% oxygen because of the additive effect of a relatively "Acidotic" pH.t; pH.