SUMMARY: As judged by the urinary excretion of free noradrenaline and adrenaline, the secretion of catecholamines is not increased in patients with Graves' disease. Catecholamine excretion was not reduced by treatment with propranolol or carbimazole singly or in combination. The possible mechanism in hyperthyroidism responsible for the apparent increase in sympathetic nervous activity and the mitigation of these features by propranolol is discussed.