Amiodarone (AMD)-induced pulmonary toxicity (AIPT) is the most life-threatening side-effect of AMD treatment. N-Monodesethylamiodarone (DEA), an active metabolite of AMD, also exhibits cytotoxicity and tends to accumulate in the lung more intensively than AMD. In this study, we characterized the mechanism of DEA accumulation using A549 cells as a model of the alveolar epithelium. Typical ATP-depletion compounds caused an approximately 30% increase in the accumulation of DEA in A549 cells, although these effects were less than those in Caco-2 cells. Triiodothyronine (T₃), which exhibited an inhibitory effect on DEA efflux in Caco-2 cells, did not affect the accumulation of DEA in A549 cells. On the other hand, 100 μM AMD caused an approximately 200% increase in DEA content in A549 cells, although AMD accumulation was not affected by 100 μM DEA. Since the reducing effect of AMD on cellular ATP levels and that of FCCP were similar, the mechanism by which DEA accumulation is increased by AMD might be different from the ATP-dependent DEA efflux mechanism. The decrease in cell viability by DEA in the presence of AMD (IC₅₀ value of DEA for A549 cell viability: 25.4±2.4 μM) was more pronounced than that by DEA alone (IC₅₀ value: 11.5±3.0 μM). This further DEA accumulation by AMD might be a factor responsible for the greater accumulation of DEA than that of AMD in the lung in long-term AMD-treated patients.