1 The steady-state plasma concentrations of metoprolol and propafenone were determined in patients being treated with one of these drugs alone and during combined treatment with both drugs. In addition, single dose studies with metoprolol, propafenone and the combination of both drugs were performed in healthy volunteers to determine the pharmacokinetics and the time course of beta-adrenoceptor blocking activity. 2 In four patients being treated with metoprolol first and subsequently with propafenone in addition steady-state levels of metoprolol increased two to five fold with simultaneous treatment with propafenone. 3 In four patients being treated with the drug combination first and thereafter with propafenone alone no changes in the steady-state levels of propafenone were observed between both treatment periods. 4 Adverse effects of the drug combination were observed in two patients (one patient experienced severe nightmares and the other left ventricular failure). 5 When single oral doses of metoprolol (50 mg) and propafenone (150 mg) and the combination of both were administered to healthy subjects, an approximately two-fold decrease of the oral clearance of metoprolol was seen when propafenone was given in addition. No conclusive changes in the pharmacokinetics of propafenone could be detected in the presence of metoprolol. 6 Duration of beta-adrenoceptor blocking activity of a single dose of metoprolol in healthy volunteers as measured by reduction of exercise-induced tachycardia increased when propafenone was given in addition. 7 The dose of metoprolol should be reduced when propafenone is given in addition.