Magnesium metabolism was studied in patients undergoing cardiopulmonary bypass. In 12 patients the mean preoperative serum magnesium level of 1.56±0.10 mEq/L fell to abnormal levels in all patients (1.07±0.04 mEq/L; P <0.001). The decrease in serum magnesium that occurred during bypass is explained by dilution of extracellular volume, as there was no significant difference between mean preoperative and bypass extracellular magnesium. The further postoperative fall in serum magnesium was less than predicted on the basis of magnesium excretion and dilution, implying movement of magnesium from intracellular to extracellular sites. Although the level of serum magnesium correlated poorly with cardiac rhythm or gross neuromuscular abnormalities, three patients with resistant ventricular fibrillation were successfully defibrillated after intravenous bolus injections of magnesium sulfate.