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The aim of the present study was to investigate the mechanisms for membrane transport of metformin in human intestinal epithelial Caco‐2 cells. The mRNA of not only organic cation transporter (OCT) 3, but also OCT1 and OCT2, was expressed in Caco‐2 cells. The uptake of 100 µm metformin at the apical membrane of Caco‐2 cells grown on porous filter membrane was significantly greater than that at the basolateral membrane. The apical uptake of 100 µm metformin in Caco‐2 cells grown on plastic dishes was inhibited significantly by 1 mm unlabeled metformin, quinidine and pyrilamine, indicating that a specific transport system is involved in the apical uptake of metformin in Caco‐2 cells. The apical uptake of 100 µm metformin in Caco‐2 cells was decreased by acidification of the medium, but not increased by alkalization. In addition, carbonyl cyanide 4‐(trifluoromethoxy)phenylhydrazone (a …