Presystemic intestinal metabolism reduces the intestinal absorption and bioavailability of orally administered drugs. The factors affecting glucuronidation activity in Caco-2 cells seeded in Transwell (4.7 cm(2)) require clarification to establish an in-vitro system to assess intestinal glucuronidation metabolism for novel drug development. alpha-Naphthol (alpha-NA), a substrate for UGT1A6 in Caco-2 cells, has often been used as a model substrate for gluruonidation. alpha-Naphthol glucuronidation activity increased from 7 to 21 culture days after seeding in Transwell and stabilized after 21 days. The higher the passage number of Caco-2 cells, the larger the variance of glucuronidation activity, but apical pH did not significantly influence glucuronidation in the pH range of 5.5 to 7.4. When the passage number ranged from 83 to 159, Km,app was highest at passage number 130. In contrast, Vmax,app increased with the passage number. This indicates that the kinetic parameters for glucuronidation in Caco-2 cells are dependent on the passage number of the cells. These results should be useful for establishing the experimental conditions for Caco-2 cells that predict intestinal glucuronidation activity in vivo.