Ethionine is a non-proteinogenic amino acid structurally related to methionine, with an ethyl group in place of the methyl group. Ethionine is an antimetabolite and methionine antagonist. It prevents amino acid incorporation into proteins and interferes with cellular use of adenosine triphosphate (ATP). Ethionine is a far more potent inhibitor of RNA synthesis than of protein synthesis. While both stereoisomers were active, the L isomer was a more potent inhibitor of RNA synthesis than was the D isomer. In male mice, 3 hr after 20 mg/kg L-ethionine, RNA synthesis was inhibited 80%, while after D-ethionine it was inhibited only 51%. Ethionine was a potent inhibitor of wool growth in sheep; the L- and D-isomers appeared equally effective. Because of these pharmacological effects, ethionine is highly toxic and is a potent carcinogen. L and D-ethionine have being shown to be equally effective in producing tissue damage in rats and mice. The L- and ID-isomers of ethionine are equally effective in producing pancreatic and renal changes in the rat as well as fatty infiltration of the liver and pancreatic damage in the albino mouse.