The antiarrhythmic effect of timolol was investigated in 160 subjects with supraventricular arrhythmias. In our double-blind, randomized, parallel, multiclinic study, subjects received timolol, 1 mg iv, or matching placebo as a starting dose, followed by a second and third dose of 1 mg each (or matching placebo) at 20-min intervals if the arrhythmia did not convert to sinus rhythm. Subjects in whom the sinus rhythm returned or the ventricular rate decreased to less than 100 bpm were transferred to a dosing regimen of timolol in 10-mg tablets twice a day by mouth, 1 hr after the last intravenous dose. Data indicated that the mean decrease in heart rate was 44 bpm after timolol and 7 bpm after placebo. The overall proportion of responders (either conversion to sinus rhythm or a decrease in ventricular rate to less than 100 bpm) was 68% after timolol and 7% after placebo. The proportions of responders after timolol were significantly higher than the proportions after placebo for paroxysmal supraventricular tachycardia (26 of 32 subjects and two of 38 subjects), atrial fibrillation (17 of 29 subjects and three of 32 subjects), and atrial flutter (seven of 11 subjects and one of nine subjects). The most common adverse effects were bradycardia and hypotension.