43 cases of pen heart surgery anesthesia done at Kyungpook National University hospital from Jun 1983 to August 1984, were measured for the concentrations of serum Na, K, Cl and CO2 before, during and after bypass. Also these values were analysed according to disease such as A(V)SD, TOF and M(S)I groups. The results were as follows. The mean values for serum sodium level before, during and after bypass were observed to be 124.41±8.62, 127.56±8.63, and 130.73±29.04mEq/G. These values revealed a tendency to elevation and the value after bypass was significantly increased comparied with the value before bypass(p<0.01). The mean values for serum potassium levels of all the cases before, during and after bypass were observed to be 3.84±1.01, 4.38±1.48, and 3.59±0.95mEq/G. The value after bypass was significantly decreased compared with the value during bypass(p<0.001). The mean values for serum chloride level of all the cases before, during and after bypass were observed to be 98.26±6.87, 97.10±6.59 and 98.35±7.94mEq/G and these values showed no significant change. The mean values for serum carbon dioxide levels of all the cases before, during and after bypass were observed to be 20.78±3.70, 22.33±4.53 and 24.87±3.72 mEq/G and these values continuously increased. The value after bypass was significantly increased. The value after bypass was significantly increased compared with the value before bypass. In the changes of serum electrolytes before, during and after bypass according to disease, the serum potassium level after bypass of the A(V)SD group and during bypass of TOF group showed a significant change(p<0.05). The changed values in the serum electrolytes before and during bypass became within normal range after bypass.