The antiarrhythmic drug, amiodarone, was administered (35 mg/kg, p.o.) once daily to lactating Fischer-344 rats for either the first 7 days of lactation, the first 14 days of lactation, or for the full 24-day lactational period. Treatment with amiodarone resulted in a decrease in maternal weight gain compared to pair-fed vehicle-treated controls, however, neonatal weight gain was not affected. The concentration of amiodarone in the maternal milk was approx. 6-fold higher than its primary metabolite, desethylamiodarone. Plasma levels of amiodarone and desethylamiodarone were higher in maternal rats compared to neonates. Both amiodarone and desethylamiodarone were distributed to neonatal tissues through lactational transfer. The levels of desethylamiodarone were higher in neonatal lung compared to liver at all treatment times. They were also elevated above amiodarone in neonatal lung at all treatment times. The amiodarone levels in neonatal lung and liver were similar.