In animals, thiamine deficiency leads to specific brain lesions, generally attributed to decreased levels of thiamine diphosphate, an essential cofactor in brain energy metabolism. However, another far less abundant derivative, thiamine triphosphate (ThTP), may also have a neuronal function. Here, we show that in the rat brain, ThTP is essentially present and synthesized in mitochondria. In mitochondrial preparations from brain (but not liver), ThTP can be produced from thiamine diphosphate and P i . This endergonic process is coupled to the oxidation of succinate or NADH through the respiratory chain but cannot be energized by ATP hydrolysis. ThTP synthesis is strongly inhibited by respiratory chain inhibitors, such as myxothiazol and inhibitors of the H ؉ channel of Thiamine (vitamin B1) is essential for brain function, and its deficiency causes specific brain lesions (1, 2). It is thought that this selective vulnerability results from decreased levels of thiamine diphosphate (ThDP), 3 an indispensable cofactor for several key enzymes in cell energy metabolism (especially pyruvate and oxoglutarate dehydrogenases and transketolase).