Although the mitochondrial Complex I has been extensively studied for over 4 decades, its catalytic properties in mitochondria are poorly understood. This review summarizes the data on standard and nonstandard kinetic parameters of the enzyme. The slow interconversion between active and deactivated forms of the mammalian Complex I (A/D transition) is described. We discuss the potential relevance of this transition for the regulation of NADH oxidation by the respiratory chain under physiological and pathophysiological conditions.