Metallothionein (MT) is a small, cysteine-rich protein that can act as a free radical scavenger at least in vitro. To test the hypothesis that MT participates in gastroduodenal cytoprotection, we studied sensitivity to gastroduodenal mucosal injury caused by ethanol in MT-null mice that have null mutations in MT-I and MT-II genes. MT-null mice and wild-type mice were orally treated with ethanol (60% or 99.5%, 0.2 ml/mouse). The macroscopic gastric lesion indices were significantly higher in MT-null mice than in wild-type mice 90 minutes after ethanol treatment. Histopathological examination in ethanol-treated MT-null mice showed vacuolar degeneration, necrosis of the epithelial cells, and hemorrhage throughout the tunica mucosa. Moreover, the duodenum also showed morphologic changes, including marked degeneration and coagulative necrosis of the entire villi, desquamation of the degenerated epithelial cells, and hemorrhage. In contrast, histopathologic changes were less prominent in the wild-type mice treated with ethanol. MT was not detected either in the stomach or duodenum of MT-null mice, whereas gastric and duodenal zinc contents were not significantly different between MT-null mice and wild-type mice. These results provide direct evidence that intrinsic MT plays a cytoprotective role in gastroduodenal mucosal injury caused by ethanol.