This study was undertaken to assess the hyperkalemic effect of several factors, including cardioplegia containing 25 mEq K+ per liter. We measured potassium balance at termination of cardiopulmonary bypass (CPB) in each of 20 patients with hyperkalemia (minimum K+, 6.0 mEq/L) and 20 patients with normal potassium levels (K+ up to 5.5 mEq/L) by subtracting urinary excretion of potassium from the sum of contributions from cardioplegia, hemolysis, and transfusion. The effects of potassium balance, diabetes, blood glucose, catecholamines, and propranolol on the degree of potassium change during CPB were assessed by multiple linear regression.