Serum Na, K, Ca and P levels were determined in 23 cases under-going open heart surgery during preoperative, early and late postoperative periods. Serum Mg values were detected in the same manner, also during extracorporeal circulation. Serum K, Na, Cl and Ca levels decreased during postoperative period. No change in serum P was observed. Serum Mg values fell during early postoperative period in 13 cases containing no Mg in the circuit, but were close to normal in 10 cases where Mg was added into the priming volume in amount of 2mEq. per 1, 000 ml. Ringer's lactate. In both groups serum Mg values were low during cardiopulmonary bypass. It has been discussed that hemodilution, hyperaldosteronism, hyperparathyroidism and diuretics might be effective in hypomagnesemia in open heart surgery. Hypomagnesemia was thought to participate in the production of postoperative arrhythmias like hypopotassemia, and thromboembolic phenomenon. importance Hence, of addition of Mg into the circuit was emphasized.