Results of studies of 15 adults placed on quinidine therapy after their serum digoxin concentrations were stabilized showed significantly increased digoxin concentrations. The average digoxin concentration before quinidine therapy was 0.75 +/- 0.28 ng/mL and after 4 days of quinidine therapy was 1.41 +/- 0.43 ng/mL. During this period, the renal clearance of digoxin decreased from 53.4 +/- 21 mL/min . 1.73 m to 35.3 +/- 12.6 mL/ min . 1.73 m. No significant correlation was found between the individual rise in serum digoxin concentrations and the rise in serum quinidine concentrations. These results suggest that serum digoxin concentration should be monitored closely for at least the first 4 days of quinidine therapy.