Abstract Administration of quinidine with digoxin increased serum digoxin concentrations in 79 patients and five volunteers. In 38 patients on a constant glycoside maintenance dose, the addition of quinidine to digoxin therapy resulted in a mean 2.5-fold increase (from 0.98±0.37 to 2.47±0.71 ng per milliliter, mean ±1 S.D.) (P<0.001). The addition of quinidine decreased renal glycoside clearance (from 91.6±27.8 to 40.6±15.8 ml per minute) (P<0.001). Unlike other investigations, our studies provided no evidence that quinidine displaced digoxin at specific cardiac binding sites. The elevated digoxin levels found during quinidine administration suggest a 30 to 50 per cent reduction of the digoxin dose. Adverse reactions to combined quinidine — digoxin therapy may be partly due to digitalis intoxication. (N Engl J Med 301:400–404, 1979)