The myocardial protection afforded by GIK solution, widely used as cardioplegic solution in this country, was compared with that provided by St. Thomas solution or oxygenated St. Thomas solution. Eighteen isolated heart-lung preparations of dogs were made and their hearts were subjected to 3 hours cold (4 degrees C) cardioplegic arrest. GIK group hearts (n = 6) received 20 ml/kg of GIK solution at the time of aortic cross-clamp perfused through the aortic root and were subsequently given 10 ml/kg of GIK solution every 30 minutes. St. Thomas group hearts (n = 6) and oxygenated St. Thomas group hearts (n = 6) were treated identically except that cardioplegic solution were St. Thomas solution or fully oxygenated one. Four hearts of GIK group showed ventricular fibrillation immediately after reperfusion that required DC countershock. Temporary A-V block was recognized in two hearts. In the other two groups, however, neither ventricular fibrillation nor A-V block was found. Heart rate, coronary flow, aortic flow and LVSW were measured before arrest and after 60 minutes of reperfusion (mean aortic pressure 70 mmHg, left atrial pressure 4 mmHg). Post reperfusion % recovery rates (post-reperfusion/before arrest) of heart rate, coronary flow, aortic flow and LVSW (mean value +/- standard deviation) were 93.4 +/- 10.32%, 104.6 +/- 24.91%, 18.8 +/- 8.54%, 32.6 +/- 6.12% respectively for GIK group, 81.4 +/- 6.50%, 125.9 +/- 15.23%, 35.4 +/- 9.91%, 56.3 +/- 12.90% for St. Thomas group and 83.1 +/- 8.40%, 121.6 +/- 16.92%, 47.0 +/- 7.89%, 69.1 +/- 9.71% for oxygenated St. Thomas group. St. Thomas and oxygenated St. Thomas groups revealed significantly (p less than 0.05, p less than 0.01 respectively) more excellent functional preservation than GIK group. Intramyocardial pH was also measured by use of glass needle pH electrode punctured into the anterior interventricular septum. Preischemic intramyocardial pH (at 37 degrees C) was 7.49 +/- 0.106 in GIK group, 7.48 +/- 0.113 in St. Thomas group and 7.43 +/- 0.114 in oxygenated St. Thomas group. During 3 hours of cardioplegic arrest, intramyocardial pH (at 4 degrees C) decreased to 6.84 +/- 0.101 in GIK group, 7.03 +/- 0.088 in St. Thomas group and 7.23 +/- 0.239 in oxygenated St. Thomas group, which was significantly higher than GIK group (p less than 0.01). Therefore oxygenated St. Thomas solution was found to maintain more favorable energy supply to ischemic myocardium. These results clearly evidenced that St. Thomas and oxygenated St. Thomas solutions would provide more effective myocardial protection during ischemic arrest than GIK solution.