Nine days after receiving a single injection of ethidium bromide - an inhibitor of mitochondrial DNA and its synthesis - mice were found to have enlarged mitochondria which were also reduced in number. The morphometric study revealed an increase of the mean mitochondrial volume, as well as an enlargement of the surface area of the mitochondrial inner membrane. However, the surface of the inner and outer membrane per unit volume of mitochondrion remained unchanged. These morphometric findings suggest mitochondrial growth, since mitochondrial inner membranes can be synthesized even in the presence of DNA-inhibiting ethidium bromide. In addition, morphometric analysis enables us to estimate the mean life span of hepatic mitochondria. In conclusion we may assume that since ethidium bromide induces not only a reduction of mitochondrial division but also an increase in the hepatic mitochondrial volume and inner membrane, the mitochondrial genome possibly fulfills a regulatory role in the mitochondrial and cytoplasmic systems for protein synthesis.