The serum digoxin concentration increased in 25 of 27 study patients (93%), and the mean serum digoxin concentration rose from 1.4 ng/ml to 3.2 ng/ml during quinidine therapy. Anorexia, nausea, or vomiting developed in 16 patients (59%) but disappeared in all ten patients for whom the digoxin dose alone was reduced, suggesting that digoxin excess caused these symptoms. Ventricular premature depolarizations developed in three patients after starting quinidine therapy; ventricular tachycardia developed in one patient, and another died suddenly. When starting quinidine therapy in patients who are taking digoxin, the clinical course, ECG, and serum digoxin level should be followed closely.