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To characterize the membrane transport responsible for the renal excretion and intestinal absorption of levofloxacin, we performed pharmacokinetic analysis of transcellular transport across LLC-PK1 and Caco-2 cell monolayers. Transcellular transport of levofloxacin in LLC-PK1 cells was greater in the basolateral-to-apical direction than in the opposite direction. Pharmacokinetic analysis indicated that basolateral uptake was the direction-determining step for the transcellular transport of levofloxacin in LLC-PK1 cells. The apical efflux clearance of levofloxacin in LLC-PK1 cells was increased at the medium pH 6 as compared with at pH 8, suggesting that membrane transport characteristics of levofloxacin are apparently similar to those of a prototypical organic cation, tetraethylammonium. On the other hand, transcellular transport of levofloxacin in Caco-2 cells was only slightly greater in the basolateral-to-apical direction than in the opposite direction. The apical efflux clearance of levofloxacin in Caco-2 cells was greater than basolateral efflux clearance, and apical influx clearance was greater than any other membrane transport clearance. In addition, the apical uptake of levofloxacin as well as quinidine in Caco-2 cells was inhibited significantly by nicotine and imipramine. The findings indicated that some transporters are responsible not only for the efflux but also for the influx of levofloxacin at the apical membrane of Caco-2 cells.