We recently purified a new thiamine diphosphate-binding protein (ThDP-BP) prepared from rat liver, and found immunoreactivity for ThDP-BP in neurons and other neuronal tissues. In this study, we examined the distribution of ThDP-BP in the rat neurons by the avidin-biotin complex technique. Strong immunoreactivity for ThDP-BP was found in the cerebral cortex, hippocampus, ependymal cells, nucleus III, nucleus V, and medial vestibular nucleus. Moderate immunoreactivity was noted in the olfactorius nucleus anterior and weak activity in the nucleus ruber. ThDP-BP may be useful in clarifying the pathogenesis of Wernicke encephalopathy and Leigh's disease.