Tumor necrosis factor α (TNF-α), which is primarily produced by macrophages, is a cytokine with various biological activities. Macrophage infiltration often accompanies experimental cholangitis in rats, and chronic cholangitis in humans. The pathophysiologic significance of TNF-α in cholangitis is not known. We used cultured, polarized intrahepatic bile duct epithelial cells (IBDECs) from rat liver to determine whether TNF-α directly affects the organization of IBDEC monolayers. The addition of recombinant TNF-α (rTNF-α) to culture media at concentrations from 10 to 200 U/mL lacked cytotoxicity to the IBDECs as judged by trypan blue exclusion and lactate dehydrogenase (LDH) release. rTNF-α transiently reduced transepithelial electrical resistance in a dose-dependent manner. During this decrease in resistance, the cellular tight junctions became leaky, allowing horseradish peroxidase (HRP) penetration. rTNF-α, at concentrations up to 200 U/mL, did not detach IBDECs from Matrigel, an artificial basement membrane. Electron microscopy and immunohistochemistry for F-actin showed a well-preserved cell structure and organization of IBDECs. Results suggest that TNF-α is nontoxic to IBDECs, and that it increases the permeability of tight junctions. TNF-α may thus disturb the barrier function of the bile duct.