Desethylamiodarone has been identified as the principal lipophilic metabolite of amiodarone present in plasma specimens from amiodarone-treated patients. This structure has been confirmed by probe-injection mass spectrometry of column effluent fractions and comparison with the authentic compound using both chromatographic and mass spectrometric techniques. Although there is no information available as to the pharmacological activity of desethylamiodarone in man, the plasma concentrations of this metabolite attained during chronic amiodarone therapy are similar to those of the parent compound (0.1-4 mg litre-1).