Potassium cardioplegia for myocardial preservation is being used extensively in open heart surgery. This study was designed to investigate the change in serum and urine potassium before, during, after the administration of cold blood potassium cardioplegic solution containing 28-30 mEq/L [28.6ulcorner.56 mEq/L] of potassium chloride in 6 dogs. Total potassium dose used in the study was 14.9ulcorner.89 mEq/L [1.2 mEq/Kg]. Mean potassium level in serum was within normal limits. Urinary excretion of potassium decreased during bypass but increased after bypass and eventually exceeded the amount of potassium infused as cold blood potassium cardioplegic solution. While cold blood potassium cardioplegia is a safe and efficient method of myocardial protection, postoperative potassium level must be observed carefully in order to detect obligatory potassium excretion following open heart surgery.