Assessment of the level of high-energy phosphates in the myocardium before, during and after ischemia was performed in 19 consecutive patients who underwent cardiac operation. The following results were obtained. 1) The levels of total nucleotides (ATP, ADP, AMP, CP) determined before, during and after aortic cross clamp were 14.94 +/- 4.12, 5.59 +/- 2.16 and 3.58 +/- 1.14 micrograms/mg. protein in the right atrial appendage. Those in the left ventricular myocardium were 18.22 +/- 4.90, 6.99 +/- 1.52 and 4.35 +/- 1.06 micrograms/mg. protein. The latter levels were higher than the former. 2) Rapidly decreasing after aortic cross clamp, the nucleotides dropped to 37-38% of preischemic level during aortic cross clamp and to 23% of preischemic level 30 minutes after reperfusion. At the termination of extracorporeal circulation, when circulatory dynamics stabilized, the nucleotides were only recovered to 50-53% of preischemic level. 3) Negative correlation was observed between the length of aortic cross clamp time and the content of nucleotides in the atrial muscle. (p less than 0.05) 4) Fluctuations in the nucleotide level indicated that the current method of myocardial protection using GIK solution produced unsatisfactory recover from ischemic damage and reperfusion injury.