Abstract In 40 consecutive patients undergoing coronary artery bypass, one of two solutions for cardioplegia, each containing 30 mEq/L of K + was used randomly. The groups were comparable except for intramyocardial temperature. With electrolyte solution (Group A), it was 16.5° ± 0.34°C, while with blood from the pump-oxygenator (Group B) it was 20.3° ± 0.41°C ( p < 0.001). After bypass left atrial pressure (LAP) was 11.9 ± 0.67 torr in Group A and 8.1 ± 0.49 torr in Group B ( p < 0.001). CPK-MB was elevated in 45% of Group A patients versus 15% in Group B ( p < 0.05). No patient died. Two myocardial infarctions occurred in Group A and one in Group B. Stereological morphometric electron microscopy was performed on biopsy specimens taken from the left ventricle (1) before perfusion, (2) after cardioplegia, and (3) 30 minutes after reperfusion. Group A showed marked intracellular edema, mitochondrial swelling, pronounced depletion of glycogen stores, and focal myofibrillary disorganization. Group B showed near normal myocardial ultrastructure with increased glycogen stores and minimal mitochondrial swelling. Morphometric analysis revealed a statistically significant increase in the degree of mitochondrial swelling (51%) in Group A compared with Group B after reperfusion ( p < 0.001). Thus, blood K + cardioplegia resulted in better preservation of myocardial ultrastructure, lower ventricular filling pressure, and lesser CPK-MB release compared with this particular electrolyte cardioplegia.