The mitochondrial DNA (mtDNA) chloramphenicol (CAP)-resistance (CAPR) mutation has been introduced into the tissues of adult mice via female embryonic stem (ES) cells. The endogenous CAP-sensitive (CAPS) mtDNAs were eliminated by treatment of the ES cells with the lipophilic dye Rhodamine-6-G (R-6-G). The ES cells were then fused to enucleated cell cytoplasts prepared from the CAPR mouse cell line 501-1. This procedure converted the ES cell mtDNA from 100% wild-type to 100% mutant. The CAPR ES cells were then injected into blastocysts and viable chimeric mice were isolated. Molecular testing for the CAPR mutant mtDNAs revealed that the percentage of mutant mtDNAs varied from zero to approximately 50% in the tissues analyzed. The highest percentage of mutant mtDNA was found in the kidney in three of the chimeric animals tested. These data suggest that, with improved efficiency, it may be possible to transmit exogenous mtDNA mutants through the mouse germ-line.