It is unknown whether amiodarone exerts a direct central action on the cardiovascular autonomic nervous system. This study was designed to evaluate the effects of acute amiodarone administration on vagal and sympathetic efferent nerve discharges. Experiments were carried out in 25 decerebrate unanesthetized rats. In one group, vagal activity was recorded from preganglionic fibers isolated from the cervical vagus nerve. In another group, sympathetic recordings were obtained from fibers isolated from the cervical sympathetic trunk in intact conditions or after barodenervation. Recordings were performed before and for 60 min after amiodarone (50 mg/kg iv) administration. In all groups, amiodarone induced bradycardia and hypotension. Vagal activity increased immediately, reaching a significant difference after 20 min (260 +/- 131% from 16.4 +/- 3.3 spikes/s) and was unmodified by the barodenervation. At difference, sympathetic activity after an initial and short-lasting increase (150 +/- 83% from 24.8 +/- 5.7 spikes/s) began to decrease significantly after 20 min (36 +/- 17%) throughout the experiment. The initial increase in sympathetic activity was not observed in barodenervated animals. These changes in vagal and sympathetic activity could play an important role in contributing to the antiarrhythmic action of amiodarone.