In a total of 18 patients, 7 females and 11 male patients with ages ranging from 23 to 70 years (mean: 45.5 +/- 14.5) diagnostic His bundle studies incorporating programmed atrial and ventricular pacing for the induction of tachycardias was performed before and after betablockade with the cardioselective betablocking agent atenolol, in a dose of 5 mg given iv. over 3 to 5 minutes. In 7 patients the pacing procedure could be repeated following ergometric exercise in order to evaluate the influence of a raised sympathetic tone on the conditions initiating paroxysmal tachycardias. At rest, atenolol prevented the pacing induced tachycardias (20 dysrhythmias in 18 patients) in 3/5 individuals with Wolff-Parkinson-White (WPW)-syndrome, in 4/6 cases with atrial tachycardias, in 4/6 patients presenting atrial flutter, in 2/2 cases developing AV-nodal tachycardias and in 1/1 individual with ventricular tachycardia. Thus, in 13 out of 19 (68%) supraventricular dysrhythmias patients benefitted from atenolol by preventing or controlling the tachycardia. Ergometric exercise changed the tachycardia or echo zone in 5/8 arrhythmias after betablockade when compared to the controls before administration of atenolol (3/5 improvement by narrowing of the tach- or echo zone, 1/5 prevention of tachycardia, 1/5 impairment due to widening of the tachycardia zone). Considering only the prevention of tachycardias, the antiarrhythmic potency of atenolol was improved in one patient with pacing induced flutter and impaired in one individual with a WPW syndrome, by ergometric exercise. These results suggest that atenolol seems to provide a good antiarrhythmic action, especially in supraventricular tachycardias, and that an increased sympathetic tone during exercise may modify the antidysrhythmic strength of betablockade.