A histochemical study concerning the activity of phosphatases and esterases of the brain has been undertaken in rats experimentally intoxicated by the fungicide ethyl-mercury-p-toluenesulfanilide (EMTS). The results have shown that compared with other mercury compounds, both organic and inorganic ones, such as corrosive sublimate and calomel, EMTS proved to be a less induced of alterations in the activity of cerebral hydrolases. The brains of animals intoxicated by EMTS revealed a notable decrease of ATP-ase and acid phosphatase activity as well as a moderate drop of AChE activity. Instead, the neuronal TPPase activity was distinctly elevated. Degenerative changes of neurons were observed in various regions of the experimental brains, the pyramidal cells of the Ammon's horn being affected most severely.