The effect of hepatotoxic dose of paracetamol (800 mg per kg, intraperitoneally, once a day during two days) on the system of microsomal oxidation, respiration, oxidative phosphorylation and high amplitude swelling of liver mitochondria was studied on 1-, 4- and 30-months old Wistar male rats. It has been shown, that paracetamol injection leads to the decrease of content of cytochrome P-450, to disorders of the function of monooxygenase system (the aminopyrine-N-demethylase and aniline hydroxylase activities were diminished), mitochondria macrostructure (the mitochondria high amplitude swelling time was decreased) and function (the respiratory control was decreased). These alterations have been observed to manifest to more extent in the liver of young rats as compared with old ones