The toxicity of bracken (Pteridium aquilinum(L.) Kuhn) to animals is complicated because this plant elaborates more than one type of agent harmful to livestock. An enzyme, thiaminase I, which destroys thiamine, is responsible for the neurotoxic syndrome. Using a radiochemical assay, the distribution of thiaminase I activity in bracken throughout the growing season has been ascertained: levels are high in the rhizome and young buds, but fall sharply in the fronds as the aerial parts of the plant unfold.The so‐called thermostable ‘antithiamine’ factors present in bracken and other plant species are discussed.The biochemical lesions of thiamine deficiency in animals are briefly outlined, and the clinical syndrome caused by the inclusion of bracken fronds or rhizomes in the diet for simple‐stomached animals (rat, horse, pig) and a ruminant (sheep) are described.All these neurotoxic syndromes respond to thiamine therapy in a dramatic way, if administered during the early stages of the