This study was designed to evaluate the effect of Z-FA.FMK (benzyloxycarbonyl-l-phenylalanyl-alanine-fluoromethylketone), a pharmacological inhibitor of cathepsin B, on the proliferation of duodenal mucosal epithelial cells and the cellular system that controls this mechanism in these cells in vivo. For this investigation, BALB/c male mice were divided into four groups. The first group received physiological saline, the second group was administered Z-FA.FMK, the third group received d-GalN (d-galactosamine) and TNF-α (tumour necrosis factor-α) and the fourth group was given both d-GalN/TNF-α and Z-FA.FMK. When d-GalN/TNF-α was administered alone, we observed an increase in IL-1β-positive and active NF-κB-positive duodenal epithelial cells, a decrease in PCNA (proliferative cell nuclear antigen)-positive duodenal epithelial cells and an increase in degenerative changes in duodenum. On the other hand, Z-FA.FMK pretreatment inhibited all of these changes. Furthermore, lipid peroxidation, protein carbonyl and collagen levels were increased, glutathione level and superoxide dismutase activity were decreased, while there was no change in catalase activity by d-GalN/TNF-α injection. On the contrary, the Z-FA.FMK pretreatment before d-GalN/TNF-α blocked these effects. Based on these findings, we suggest that Z-FA.FMK might act as a proliferative mediator which is controlled by IL-1β through NF-κB and oxidative stress in duodenal epithelial cells of d-GalN/TNF-α-administered mice.