Adenosine, verapamil, propranolol, and procainamide are widely used antiarrhythmic drugs. The interactions among them are still not known in human beings. Adenosine-induced negative dromotropic effects were assessed by rapid bolus injection of adenosine during constant high right atrial pacing in each patient. The initial dose of adenosine was 0.5 mg and was followed by a stepwise increment of 0.5 mg until atrioventricular (AV) nodal block occurred. The negative dromotropic actions of adenosine were examined in the control state and in the following three protocols in three groups of patients: (1) In 12 patients (group 1), intravenous verapamil, 0.15 mg/kg, was given; (2) In 12 patients (group 2), intravenous propranolol, 0.1 mg/kg, was given; and (3) in 10 patients (group 3), intravenous procainamide, 15 mg/kg, was given. The dose-response curves of adenosine on AV nodal conduction were almost identical in the control state and after verapamil, propranolol, or procainamide injection. However, verapamil, in contrast to propranolol, significantly reduced the dose of adenosine required to produce AV nodal block, from 4.4 +/- 0.7 mg to 2.7 +/- 0.3 mg (p < 0.01). Of note, procainamide exerted no significant effects on adenosine-induced negative dromotropism on AV nodal conduction or AV nodal block. In conclusion, the negative dromotropic effects of adenosine are preserved and independent even in the presence of verapamil, propranolol, or procainamide. Both verapamil and propranolol can exhibit additive effects with adenosine in prolonging AV nodal conduction time; however, only verapamil can reduce the dose of adenosine required to produce AV nodal block. This finding indicates that the dose of adenosine may be reduced for patients who have already been treated with verapamil.