Sotalol, a beta-adrenergic blocking agent devoid of intrinsic sympathicomimetic activity and local anaesthetic properties, was assessed in a double-blind cross-over study for the symptomatic treatment of 10 patients with thyrotoxicosis. The heart rate and clinical thyrotoxicosis index were significantly lower than with the placebo or with no treatment. The values of free thyroxine and triiodothyronine in the serum did not change. Because of its pure beta-adrenergic blocking effect, sotalol is suitable for the symptomatic treatment of thyrotoxicosis.