The hepatotoxic action of allyl formate on rat liver has been investigated. Biochemical changes can be detected in the liver cell many hours before the histological changes and it would appear that the toxin has a direct action on the liver parenchymal cell. The results suggest that allyl formate is not the toxic agent but that it is converted via allyl alcohol into acrolein. This reaction requires the presence of alcohol dehydrogenase. Histochemical studies have shown that this enzyme is localized in the periportal region of the liver lobule, and may explain why allyl formate solely produces a periportal necrosis. As glutathione and 1,4-dithiothreitol protect against the early biochemical changes produced by the poison, it is probable that acrolein alkylates proteins and nucleic acids.