Potassium cardioplegia for myocardial preservation is being used extensively in heart operations. This study was designed to determine the effect of potassium cardioplegia on serum and urine potassium levels. A control group of 11 patients was compared to a study group of 24 patients. Myocardial preservation in the control group was achieved by whole body cooling to 20 degrees to 30 degrees C and in the study group, by repeated injections every 30 minutes of 500 to 700 ml of cold pump blood, containing potassium chloride 30 mEq/L, into the aortic root after aortic clamping. Total potassium dose in the study group was 46 +/- 21 mEq (mean +/- SD). Mean serum potassium level was significantly higher during and after bypass in the study group (after bypass: control 3.65 +/- 0.11 mEq/L, study 4.24 +/- 0.10 mEq/L [mean +/- SE], p less than 0.005), but was within normal limits in both groups. Urine potassium levels and excretion rates were significantly higher in the study group (potassium excretion rate after bypass: control 6.1 +/- 0.7 mEq/hr, study 11.3 +/- 0.9 mEq/hr [mean +/- SE], p less than 0.0025). We conclude that no special measures are required to facilitate potassium excretion when total potassium cardioplegia dose is not greater than 50 mEq.