To evaluate an in vitro model suitable for investigating intestinal first-pass drug metabolism, CYP3A4 and MDR1 mRNA induction by 1alpha,25-dihydroxyvitamin D(3) (VD3) was examined in two human intestinal cell lines, Caco-2 and LS180, under various culture conditions. CYP3A4 mRNA expression was induced by 100 nM VD3 at levels between 234-549 times above normal in Caco-2 cells for 2 weeks and by 74-200 times above normal in LS180 cells for 2 days. The CYP3A4 induction effect of 250 nM VD3 was similar to or slightly higher than that of 100 nM VD3 in both Caco-2 and LS180 cells. Also, CYP3A4 was induced in Caco-2 and LS180 cells when they were cultured on a polystyrene plate slightly less than when they were cultured on a porous membrane. The increase in fetal bovine serum (FBS) content in the culture medium resulted in little or only slight increase of CYP3A4 induction in both Caco-2 and LS180 cells. MDR1 mRNA expression was marginally increased by VD3 in LS180 cells, but not in Caco-2 cells, and neither increased FBS content nor use of a porous membrane significantly affected MDR1 induction in LS180 cells. The transepithelial electrical resistance of LS180 cells was almost zero, whereas that of Caco-2 cells was high and was marginally decreased by VD3. These findings indicate that Caco-2 cells cultured on a porous membrane with 100 nM VD3 for 2 weeks may be used as a model to investigate the intestinal absorption and first-pass metabolism of drugs, while LS180 cells may be utilized to elucidate the mechanisms which regulate intestinal CYP3A4 mRNA expression.