The present study was carried out to develop the better measures for safety of open heart surgery under extracorporeal circulation [ECC] with Heart-Lung-Machine by preventing changes in the concentrations of serum electrolytes during and after ECC. For this purpose,the concentrations of serum electrolytes were measured before, during, and after ECC in 21 patients with congenital and acquired heart diseases who received open heart surgery under EGG using Heart-Lung-Machine. Also considered was the development of safety measured by which changes in serum electrolyte concentrations were prevented during and after open heart surgery under ECC. 1] The mean values for serum sodium levels were observed to be 137.14?.47 mEq/L for the samples obtained before ECC, 138.59?.68 mEq/L for the samples obtained 10 minutes after ECC and 138.0?.68 mEq/L for the samples obtained 24 hours after ECC. These results indicate that serum sodium concentrations were within normal range during and until 24 hours after ECC. 2] The concentrations of serum chloride were found to be 105.38?.70 mEq/L for the samples collected before EGG, 105.07?.01 mEq/L for the samples collected 10minutes after EGG and 101.95?.09 mEq/L for the samples collected 24 hours after ECC. As was the case with serum sodium levels, no significant changes were observed in serum chloride levels during and 24 hours after ECC. 3] With proper provisions of potassium chloride solution during EGG, the concentrations of serum potassium were found to be 4. 22?.06 mEq/L for the samples removed before ECC, 4.06?.14 mEq/L for the samples removed 10 minutes after EGG and 4.39?.07 mEq/L for the samples removed 24 hours after ECC, 4] The concentrations of serum calcium were also maintained within normal during and after ECC; 9.15?.14 mg/dl for the serum collected before ECC, 8.36?.21 mg/dl for the serum collected 10 minutes after EGG and 8.47?.14 mg/dl 24 hours after ECC. The maintenance of serum calcium level within normal throughout ECC was achieved by parenteral administrations of calcium gluconate as frequent as required. 5] As were the cases with serum potassium and calcium, the concentrations of plasma bicarbonate was regulated within normal range during and after ECC, only when sodium bicarbonate solution was administered parenterally as it was required; 23.7?.50 mEq/L for the serum collected before ECC, 22.33?.09 mEq/L for the serum collected 10 minutes after ECC and 25.3?.96 mEq/L for the serum collected 24 hours after ECC. The above results indicate that during and after ECC serum sodium and chloride levels remained unchanged without any provision of normal saline, while serum potassium, calcium, and bicarbonate concentrations were kept within normal limits only when these electrolytes were administered through parenteral routes. With these results it can be concluded that serum potassium, calcium, and bicarbonate levels should be determined as often as possible during and after ECC and that in order to maintain serum electrolyte levels within normal these electrolytes in the forms of potassium chloride, calcium gluconate, and sodium bicarbonate should be given parenterally as they were found to be required.[KTCS 1978;4:404-415]