The unique features of mtDNA, together with the lack of a wide range of mouse cell mtDNA mutants, have hampered the creation of mtDNA mutant mice. To overcome these barriers mitochondrial defects were created by introducing mitochondria from different mouse species into Mus musculus domesticus (Mm) mtDNA-less (rho(0)) L cells. Introduction of the closely related Mus spretus (Ms) or the more divergent Mus dunni (Md) mitochondria resulted in xenocybrids exhibiting grossly normal respiratory function, but mild metabolic deficiencies, with 2- and 2.5-fold increases in lactate production compared with controls. The transfer of this model from in vitro to in vivo studies was achieved by introducing Ms and Md mitochondria into rhodamine-6G-treated Mm mouse embryonic stem (ES) cells. The resultant xenocybrid ES cells remained pluripotent, and live-born chimerae were produced from both Ms and Md xenocybrid ES cells. Founder chimeric females (G(0)) were mated with successful germ-line transmission of Ms or Md mtDNA to homoplasmic G(1) offspring. These xenocybrid models represent the first viable transmitochondrial mice with homoplasmic replacement of endogenous mtDNA and confirm the feasibility of producing mitochondrial defects in mice by using a xenomitochondrial approach.