During the period of August 1983 to February 1984 study has been done on the value of serum lactate at regular intervals before, during and after operation on 30 patients who went through open heart surgery with extracorporeal circulation for congenital or acquired heart diseases at the Department of Thoracic and Cardiovascular Surgery, School of Medicine, Hanyang University- The results were as follows: 1.The mean value of serum lactate before the operation was observed to be 28.5 ?.41 mg/dl. 2.At 10 minutes after the beginning of extracorporeal circulation the mean value of serum lactate showed a rapid increase to 73.24?3.61 mg/dl, an increase of 44.74 mg/dl [156.9%] from the pre-operation value. 3.At 40 minutes after the beginning of extracorporeal circulation the mean value of serum lactate was observed to be 78.98?1.67 mg/dl which was the highest level. It was an increase of 50.48 mg/dl [177.12%] from the preoperation value. 4.At 70 minutes after the beginning of extracorporeal circulation the mean value of serum lactate was observed to be 64.39?9.29 mg/dl, an increase of 35.89 mg/dl [125.9%] from the preoperation value, and at over 100 minutes it was observed to be 68.00?5.79 mg/dl, an increase of 39.5 mg/dl [138.5%] from the preoperation value. 5.The mean value of serum lactate immediately after the operation was 61.36?6.94 mg/dl, an increase of 32.86 mg/dl [115.2%] from the preoperation value. 7.On the 7th post-operative day the value of serum lactate returned the preoperation value.