Two pathways of free oxidation in liver mitochondria were examined. One of these pathways is determined by the protonophoric action of free fatty acids, and the other pathway, by passive proton leakage in the absence of fatty acids. According to the model of the proton futile cycle of mitochondria, the protonophoric activity of fatty acids was defined as a quotient of the division of the acceleration of respiration by fatty acid by the coefficient of respiration control for the proton leakage. The temperature dependence of the palmitate protonophoric activity on the Arrhenius plot has a break at 22 degrees C and is characterized by the transition of activation energy from 120 to 60 kJ/mol. The dependence of the respiration rate in state 4 on the Arrhenius plot is linear and, the activation energy is 17 kJ/mol. It was concluded that the first pathway of free oxidation is determined by the cyclic transport of fatty acids with the participation of metabolic carriers, and this process depends on the membrane fluidity; the second pathway is determined by passive leakage of protons through membrane channels, without fatty acids and this process is independent on membrane fluidity.