In many studies on succinate as a substrate, the respiration of brown adipose tissue mitochondria is partially uncoupled that is associated with proton leak. NAD-dependent substrates (pyruvate + malate) were also used in a number of works. Oxidation of NAD-dependent substrates occurs with greater coupling where the respiration rate is higher and state 4 (resting state of mitochondrial respiration) lower than that with succinate. It is suggested that the oxidation of these substrates occurs in different ways.