The effect of propranolol (40 mg t.d.s.) on the peripheral levels of T4, T3 and reverse-T3 was studied in 26 patients with hyperthyroidism. The compounds were measured by specific radioimmunoassay techniques. The levels were followed for five weeks and compared with the levels before treatment. The results show that propranolol had no significant effect on the peripheral levels of T4 despite a rapid amelioration of clinical symptoms. A significant reduction of T3 levels was obtained during the first to fourth week of treatment. No significant decrease was obtained after the fourth week. A significant elevation in reverse-T3 levels was obtained during the second to fourth week of treatment. No significant change was obtained either during the first week or after the fourth week. Inconsistent fluctuations in hormone levels, both before and during treatment, could be seen in individual cases. The results suggest that the reduction of clinical symptoms must be caused by an extrathyroidal action of propranolol which does not seem to involve the pituitary gland.