Sexual dimorphisms of atherosclerosis and the susceptibility to arrhythmias and antiarrhythmic treatment have been reported. This study investigated acute effects of amiodarone on endothelium-dependent relaxation in the aorta of male and female apoE0 mice with advanced atherosclerosis. Amiodarone tissue uptake was quantified by high-performance liquid chromatography, and xanthine oxidase-dependent superoxide anion formation was investigated in vitro in presence or absence of amiodarone. Incubation with amiodarone for 30 min improved endothelium-dependent relaxation, which was associated with rapid vascular accumulation of amiodarone (P < 0.001) that was sex-dependent. In males, reduced endothelium-dependent relaxation was improved by amiodarone (from 88 +/- 3% to 100 +/- 2%, P < 0.01). Spontaneous phasic contractions, which were greater in females than in males (P < 0.001), were completely abolished by amiodarone (P < 0.0001). Amiodarone also inhibited generation of superoxide anion (P < 0.0001). These data show that amiodarone rapidly accumulates in atherosclerotic vascular tissue, abolishes vascular autorhythmicity, and improves endothelium-dependent function in atherosclerotic arteries. Antioxidant and vasodilator effects following amiodarone administration may contribute to its antiarrhythmic effects.