Mitochondria perform two critical functions 1) the production of more than 90% of the cell’s energy &, 2) control of cell survival as a part of programmed cell death (apoptosis). Assessing test compounds for mitochondrial toxicity is important in drug safety evaluation, with increasing focus early on in the development process. Mitochondrial toxicants have little e ect on cell growth or viability in glucose-fed cells. Immortalized cell lines are metabolically adapted for rapid growth under hypoxic & acidic conditions, & derive almost all of their energy from glycolysis rather than via mitochondrial oxidative phosphorylation (OXPHOS), the Crabtree effect. In galactose-fed cells, oxidation of galactose to pyruvate via glycolysis yields no net ATP, therefore cells are forced to rely on mitochondrial OXPHOS to generate sufficient ATP for survival. Replacing media glucose with galactose increases the susceptibility of HepG2 Cells to mitochondrial toxicants.