Quinidine elevates serum digoxin concentration in part by reducing the volume of distribution of digoxin, which implies that quinidine displaces digoxin from tissues. The purposes of this study were to: 1) measure the effect of quinidine on tissue digoxin concentrations, and 2) determine if quinidine alters the relation between myocardial digoxin concentration and digoxin effect on myocardial monovalent cation transport. Eighteen dogs were treated with tritiated digoxin until the steady-state serum digoxin concentration was between 1.0 and 1.5 ng/ml. All dogs continued receiving the same dose of digoxin while nine dogs were given quinidine as well. Quinidine was continued until the serum digoxin concentration had increased by at least 25%. At the end of treatment, the serum digoxin concentration in dogs treated with digoxin was 1.2 +/- 0.1 ng/ml compared with 2.1 +/- 0.5 ng/ml in dogs treated with digoxin and quinidine in combination (p less than 0.001). Digoxin concentration in myocardium, skeletal muscle, liver, kidney, stellate ganglion, vagus nerve, femoral nerve, brain and brainstem medulla was higher in dogs treated with a combination of digoxin and quinidine than in dogs treated with digoxin alone, but remained proportional to the serum digoxin concentration in all tissues except the brainstem medulla. Myocardial monovalent cation transport was measured using rubidium-86. The effect of digoxin on myocardial monovalent cation transport did not increase as the serum and myocardial digoxin concentrations increased after quinidine administration.