To determine whether a single dose of quinidine sulfate might alter serum digoxin levels, patients receiving maintenance digoxin therapy who required quinidine had their serum digoxin levels measured after oral administration of digoxin, quinidine, or both. In group 1, digoxin and quinidine were administered orally together; in group 2, digoxin was administered orally alone; and in group 3, quinidine was administered orally, 24 hours after the last dose of digoxin. In group 1, serum digoxin level increased by 2.0+/-0.4 ng/mL (from 1.0+/-0.1 ng/mL), but in group 2, serum digoxin level increased by only 1.0+/-0.1 ng/mL (from 1.0+/-0.1 ng/mL). Group 3 showed no change in serum digoxin concentration. Time of maximum digoxin concentration also occurred earlier when digoxin and quinidine were administered together. Thus, a single dose of quinidine will increase the peak digoxin level when administered together with oral digoxin.