1 Administration of isoprenaline by aerosol inhalation to man results in over 80% being metabolized to the sulphate conjugate.2 The majority of an inhaled dose is probably swallowed since the metabolic pattern resembles that after an oral dose.3 Isoprenaline, administered in aqueous solution directly into the bronchial tree in both man and dog, is rapidly O-methylated to 3-O-methyl isoprenaline, which is subsequently conjugated with sulphate.4 3-O-Methylation is the main metabolic pathway for the small part of an inhaled dose which does enter the bronchial tree.