Serum, red blood cell, and atrial potassium concentrations were measured in 148 patients undergoing cardiac surgery. No significant difference was noted between serum and red cell potassium concentration and atrial arrhythmias. Seventy-six patients were in sinus rhythm prior to and following operation and had an average atrial potassium concentration of 38.2 mEq/100 g. Thirty patients in sinus rhythm developed atrial flutter or fibrillation after surgery, and the average atrial potassium concentration was 30.4 mEq/100 g. Forty-two patients were in atrial fibrillation before operation and had an average atrial potassium concentration of 26.6 mEq/100 g. A direct relationship existed between chronic atrial fibrillation and atrial potassium concentration. All developing postoperative atrial fibrillation had a reduced atrial potassium concentration in the preoperative biopsy. However, nine patients had reduced potassium concentration but did not develop atrial fibrillation.