The mechanisms of action of the potent antiarrhythmic drug amiodarone are unknown. However, amiodarone and its abundant metabolite, desethylamiodarone, bear a striking structural resemblance to thyroid hormones. In addition, certain cardiac electrophysiologic effects of amiodarone treatment are similar to those of hypothyroidism. These facts suggest that amiodarone or desethylamiodarone could be acting, in part, by blocking thyroid hormone action. Because thyroid hormones are known to act through nuclear receptor prqteins, the binding of amiodarone and desethylamiodarone was measured to nuclear extracts derived from human lymphocytes, bovine atrium and ventricle and rat liver. The capacity of increasing concentrations of amiodarone and desethylamiodarone nuclear extracts to block receptor binding of radiolabeled triiodothyronine (T3) in a standard in vitro competition assay was tested. Nuclear extracts demonstrated only minimal binding to amiodarone. However, all receptor preparations had substantial affinities (KD) for the desethyl analog: lymphocyte, 8.6 μM; atrium, 35.0 μM; ventricle, 26.9 μM and liver, 8.6 μM. Desethylamiodarone accumulates in very large quantities in parenchymatous organs during long-term amiodarone treatment. Taking its usual therapeutic serum level (about 4 μM or 2.7 μg/ml) as an estimate of intranuclear concentration, desethylamiodarone would partially saturate nuclear thyroid hormone receptors in several different tissues, including the heart. Thus, amiodarone treatment may exert some of its electrophysiologic effects by metabolic conversion to desethylamiodarone. This metabolite may then exclude thyroid hormone from nuclear receptor sites within the myocardium.