Abstract: The levels of magnesium in serum,plasma,erythrocyte and urine were determined in 11 cases undergoing open heart surgery,Samples were taken daily before,during and after cardi-opulmonary bypass operation for 9 days.Results showed that the amount of magnesium supplementduring and after operation may prevent and correct hypomagnesemia syndrome.It is reasonable toapply a programm of magnesium supplement during the period for patients undergoing open heartsurgery,This programm may be used in other hospitals.