(2-Bromoethyl)ethyl(naphth-1-ylmethyl)amine hydrobromide (SY28) is a halogenoalkylamine related to dibenamine. A dose of 10 mg/kg injected intravenously into rats antagonizes the pressor response to adrenaline for 36 hr. If this amount of SY28 labelled with (14)C in the 1-methyl position is administered, specific radioactivity is present in blood and tissues many days after the antagonism of adrenaline is relieved. The (14)C is excreted in bile and in urine, but not in expired air. It is present in fat but not to a greater extent than it is in other tissues. It does not cross the placental barrier. There is no evidence that slow release from a lipid depot accounts for the long duration of action.