Arrhythmias were analyzed in 50 patients undergoing cardiac surgery: 27 with valve surgery, 15 with coronary artery bypass (CAB), 5 with CAB and valve surgery, and 3 with miscellaneous procedures. The role of electrolyte abnormalities, pericarditis, serum osmolarity, digoxin level, and the type of surgery performed was evaluated. Thirty-seven out of 50 patients (74 per cent) had a postoperative arrhythmia, and a total of 78 different arrhythmias were noted. Twenty-six out of 27 patients with valve surgery had an arrhythmia vs. six out of 15 patients with CAB (p less than 0.001). Atrial fibrillation was the most common arrhythmia in all groups. Although postoperative hypocalcemia, hypomagnesemia, pericarditis, and wide shifts in osmolarity were common, they did not correlate with arrhythmias. Seventeen patients developed postoperative arrhythmias compatible with digitalis toxicity, including junctional rhythm, atrioventricular dissociation, or atrial tachycardia with block. However, the range of serum digoxin levels in these patients was zero to 2.80 ng. per milliliter. This suggests increased sensitivity to digitalis glycosides or the effects of surgical trauma as the etiology of arrhythmia in many patients. The distinction between digitalis-induced arrhythmia and spontaneously occurring arrhythmia cannot be made with certainty in most postoperative patients. Therapy should reflect an awareness of the potential for postoperative digitoxicity.