Controlled metabolic studies were used to gauge the relative efficacy of two cardioplegic solutions in 28 patients (14 in each group) undergoing multiple coronary artery bypass grafts. A solution containing magnesium-potassium (Plegisol) was compared to a standard potassium crystalloid cardioplegic solution. Measurements of coronary blood flow, coronary vascular resistance, coronary arteriovenous oxygen difference, myocardial oxygen consumption and extraction, and myocardial lactate and potassium extraction and release were all measured in the isolated, vented, paced, beating heart, before and for 15 minutes after a one hour arrest interval during which time revascularization was completed. During cardioplegic administration, the infusion flow rate, myocardial oxygen consumption and extraction, and lactate and potassium release and uptake were noted. The results indicate that during cardioplegic administration, the total oxygen consumed for both potassium and magnesium-potassium solutions did not significantly differ. The flow rate of the Mg-K solution was significantly higher than that of the potassium solution alone (510 vs. 398 ml/min). There was no lactate production during Mg-K administration, but 0.13 mEq/min of lactate was produced while potassium crystalloid cardioplegia was given. During myocardial reperfusion, oxygen extraction was maintained near prearrest levels in both groups. The only significant difference noted between the potassium and magnesium-potassium solutions were the higher coronary blood flow and oxygen consumption immediately upon reperfusion in the Mg-K group.(ABSTRACT TRUNCATED AT 250 WORDS)