Serum triiodothyronine (T3) and thyroxine (T4) levels were measured in twelve hyperthyroid patients before and after treatment with propranolol, 40 mg four times daily, for 2 weeks. There was a significant fall in serum T3 and a significant rise in serum T4 concentrations in the group as a whole and it was concluded that the clinical effectiveness of propranolol in hyperthyroidism may be mediated in part by its action on the peripheral metabolism of thyroid hormones. Propranolol treatment should be withdrawn gradually as removal of the suppressive action of the drug on thyroid hormone metabolism is potentially hazardous, particularly in patients with ischaemic heart disease.