Mitochondria are organelles responsible for Adenosine triphosphate (ATP) production and are central in biological aging research. Laboratory interventions that extend healthy animal lifespans also work to treat severe animal mitochondrial disease, however, how these interventions work at the molecular level is still unknown. Several longevity interventions extend lifespan and treat a model of Leigh Syndrome, a severe mitochondrial disease, in mice. Among these, rapamycin inhibits the metabolic master regulator mTOR (mechanistic target of rapamycin). This evidence suggests that mitochondrial disease and biological aging share a common cause at the cellular level. When mTOR is inhibited, SIRT3 is upregulated, an enzyme that controls mitochondrial fatty acid oxidation (FAO). We are using the mouse model of Leigh Syndrome, Ndufs4 knockout (KO) mice, to ask the following question: Does rapamycin treatment require SIRT3 activity to increase FAO to treat mitochondrial disease? Preliminary results show that SIRT3 is required for lifespan extension in Ndufs4 KO mice with rapamycin treatment. SIRT3 has also been observed regulating FAO. We are using both etomoxir, a drug that inhibits FAO, and rapamycin to answer this question by measuring lifespan. We are also measuring mitochondrial FAO directly in Ndufs4 Sirt3 double KO animals. Our results are allowing us to better understand how mitochondrial disease and normative aging are related, which will streamline targeting the biology of aging and mitochondrial dysfunction in humans.