Alterations in the serum and urine potassium were studied in twenty patients who underwent open heart surgery in National Medical Center during the period from Jan.1978 to June 1978. There were twelve cases congenital heart disease and eight acquired heart disease. Rigg-Kyvsgaad mark IV roller pump and Polystan bubble oxygenator were used in all patients. Hemodilution was carried out by priming the oxygenator with Hartmann`s solution. Measurements were made of the serum electrolyte, gas analysis and twenty four hour urine electrolytes. During the bypass, the serum potassium decreased Significantly from 4.2?.47 to 3.6?. 72 mEq. per liter. [p<0.05] Fifty seven miliequivalant of potassium chloride were added during operation-urine potassium was higher in the diuretic group than in the non diuretic group. Plasma potassium level in the diuretic group was more significantly reduced than nondiuretic group. In this series large amount of urine potassium loss was noted on the day of operation, the first and second post operative day.[KTCS 1978;4:422-427]