Studies of monoxygenase system in rat liver exposed to tricresol showed, that the microsomal enzymes status depends on the route of tricresol administration and on the exposure intensity. Monoxygenase system status compared with the serum enzymes activity characterizing the function of the liver showed that changes in microsomal system are more marked, appear earlier and after lower levels of exposure. Changes of cytochrome P-450 enzyme system are the main principle of tricresol harmful action.