Mitochondrial respiration in cancer cells is more uncoupled from ATP production than that in nontransformed cells. Aerobic glycolysis and mitochondrial respiration were quantified in murine breast cancer cells (NT2196) and parental controls (NMuMG) as well as in human breast cancer cells (MCF7) and nontransformed controls (MCF10A). (A) Glucose consumption and (B) lactate production in cancer cells are presented as fold change from controls. (C) Total mitochondrial respiration, (D) coupled respiration and (E) uncoupled respiration in cancer cells are presented as fold change from controls. (F) The fraction of mitochondrial respiration devoted to coupled and uncoupled respiration was calculated by dividing the rate of coupled or uncoupled respiration by that of total mitochondrial respiration. Coupled respiration is the respiration used to drive ATP synthesis. Uncoupled respiration is used to drive proton leak reactions. Data are presented as means ± SEM. n = 3. *P <0.05, Students t- test, where * represents a significant change from nontransformed controls.