Hypothyroid, hyperthyroid and euthyroid rats were given 45 or 80 mg/kg i.v. or 100 mg/kg p.o. of amiodarone hydrochloride to determine the effects of thyroid dysfunction on the absorption and disposition characteristics of amiodarone. Serial blood samples were obtained for 48 hr and assayed for amiodarone and desethylamiodarone by high-performance liquid chromatography. In the hypothyroid rats, reductions in amiodarone clearance (CL) of 73% (26.9-7.3 ml/min/kg) and 61% (18.7-7.3 ml/min/kg) were observed with the 45- and 80-mg/kg i.v. bolus doses, respectively. Accompanying the decreases in CL were increases in the terminal disposition half-life (T1/2 gamma), 89% (18-34 hr) with the 45-mg/kg dose and 185% (20-57 hr) after the 80-mg/kg dose. The steady state (Vss) and apparent (Vd) volumes of distribution were smaller at the lower dose but were invariant after administration of the larger dose. Furthermore, the central compartment volume was not altered. In the hyperthyroid rats, a 67% increase in CL (12.8-21.4 ml/min/kg) and 75 to 80% increases in Vss (15.5-27.1 liters/kg) and Vd (25.0-44.8 liters/kg) were observed with the 45-mg/kg of amiodarone dose. However, no changes in CL, Vd and Vss were seen with the 80-mg/kg dose. Furthermore, gamma, T1/2 gamma and central compartment volume were not altered in the hyperthyroid rats. The effects of thyroid dysfunction on the p.o. bioavailability characteristics of amiodarone were minor. These studies demonstrated that the disposition kinetics of amiodarone are altered in hypo- and hyperthyroidism.