Amiodarone has a good antiarrhythmic effect administered either acutely or chronically. Since the antiarrhythmic effect of chronically administered amiodarone has been thought to be dependent on a depression of thyroid function, we studied the peripheral hormonal pattern of 10 euthyroid patients with ventricular arrhythmias who had been responsive to the acute intravenous administration of the drug (10 mg/Kg). During the first 12 hours following the drug administration, reverse T3, free T3 and free T4 values and QTc duration were unchanged. Therefore the antiarrhythmic effect of amiodarone when acutely administered has no correlation with thyroid hormone serum changes.