Effects of methyltriclosan, a biodegradation product of the antimicrobial agent triclosan, on the functional parameters of rat liver mitochondria were studied. It was found that methyltriclosan inhibits the activity of complexes II and IV of the respiratory chain. This causes a decrease in the rate of oxygen consumption by mitochondria in states 3 and 3UDNP. Methyltriclosan was shown to increase the rate of hydrogen peroxide production by rat liver mitochondria. In addition, methyltriclosan induces permeabilization of both mitochondrial membranes and the liposome membranes. Methyltriclosan, in contrast to triclosan, has a milder effect on organelles and their membranes. Possible mechanisms of methyltriclosan effects on mitochondria and liposomes are discussed.