Ethionine, the ethyl analogue of methionine, is reported to be toxic in several species, including rats (Farber 1963). Most of the effects after oral or parenteral exposure of ethionine are acute, occurring within hours or a few days, but others, such as liver cancer, appear only after chronic administration for several months (Farber 1963). Tiadenol is a potent peroxisome proliferator (Berge and Bakke 1981) and this drug, as well as ethionine, is characterized as a nonmutagenic carcinogen (Lillehaug et al. 1986). Peroxisome proliferators constitute a novel class of nonmutagenic carcinogens, all of which induce similar pleiotropic responses consisting of hepatomegaly, proliferation of peroxisomes in the liver parenchymal cells, and the induction of several hepatic enzymes, especially those of the peroxisomal β-oxidation (Reddy and Lalwani 1983). It is of particular interest that the induction of peroxisomes by peroxisome proliferators is associated with a dramatic increase in the ratio of peroxisomal β-oxidation to cat-alase, leading to increased generation of hydrogen peroxide and oxidative stress (Reddy and Lalwani 1983).