Metallothionein (MT) is a small, metal-binding protein that can act as a scavenger of free radicals. To determine whether MT is involved in ethanol-induced hepatotoxicity, which is known to occur through oxidative stress, we studied sensitivity to hepatotoxicity caused by ethanol in MT-null mice genetically deleted for MT-I and MT-II. MT-null mice and wild-type mice were i.p. administrated with ethanol (99.5%, 2.0 g/kg). The increase in GPT, GOT, and LDH activities in the serum of MT-null mice was significantly higher than in the wild-type mice 24 hr after ethanol treatment. Histopathological examination in the liver of ethanol-treated MT-null mice demonstrated vacuolar degeneration. In contrast, histopathologic change was less prominent in the liver of ethanol-treated wild-type mice. Moreover, ethanol increased lipid peroxidation levels only in the liver of MT-null mice. These results indicate that deletion of MT is associated with ethanol-induced severe hepatotoxicity through oxidative stress.