Repeated oral treatment of dogs with a high dose of nefiracetam is reported to induce hemorrhagic lesions in the urinary bladder. To delineate its pathogenesis, we established the primary culture of uroepithelial cells of the canine urinary bladder, and then explored the effect of nefiracetam on the cultured cells. Uroepithelial cells scraped from the connective tissues of the urinary bladder of naive dogs were suspended in the minimum essential medium containing dispase, and then resuspended in the keratinocyte medium to be 6.0-7.0 x 10(5) cells/ml. Afterward, they were added to an apical chamber with a 12-mm transwell filter, cultured for three days, and recultured in the keratinocyte medium containing 1 mM CaCl(2) for 20 days. Microscopically, these cultured cells consisted of three cell layers with high transepithelial electric resistance (TER; > 10,000 ohm-cm(2)). Immunofluorescence observations revealed ZO-1 and E-cadherin bands, and electron microscopic examinations displayed the superficial cells with the assembly of tight junctions. When the effect of nefiracetam and its five main metabolites (M-3, M-10, M-11, M-18, and M-20) on TER and the ZO-1 band was assessed using cultured cells, only M-18 significantly reduced TER in the coculture for 48 h or more. Both M-10 and M-18 exhibited a deformation of uroepithelial cells and a slight reduction of the ZO-1 band from 120 h later. In conclusion, this culture system possesses both functional and morphological features of the uroepithelium reflected in vivo, and M-18 may play a pivotal role in the impairment of uroepithelial cells, leading to the onset of the urinary bladder lesion in dogs due to nefiracetam.