ICI 50,172 is a cardioselective beta-adrenergic receptor blocker. A dose of 20 mg was given intravenously to each of 75 atropinized patients lightly anaesthetized with halothane in oxygen. Adrenergic overactivity of the heart was present in each patient before the drug was administered. The drug blocked the chronotropic, inotropic and dysrhythmic reacton of the heart to the adrenergic stimuli provoked by surgery under halothane anaesthesia. It did not influence dromotropic activity. It is concluded that ICI 50,172 is superior to propranolol in anaesthetized patients because of its cardio-selectivity and its lack of analgesic action.