Some aspects of reactive nitrogen and oxygen species (RNS and ROS) metabolism in rat heart mitochondria under admin­istration of different doses of nitroglycerine (NG) in vivo are discussed. It is shown that NG administration results in a dose-dependent increase in Ca2+-uptake in mitochondria, due to the dose-dependent inhibition of mitochondrial permeabil­ity transition pore (MPTP) in vivo and the activation of Ca2+-dependent mitochondrial NOS. It was shown that NOS activ­ity increases in accord with the increase of Ca2+-uptake in mitochondria. The dose-dependent activation of nitrat-reductase is observed. However, nitrite production decreases dose-dependently, according to the change of NO2-/NO3- ratio on behalf of NO3-, the end product of NO transformations. The relation between nitrosylation of mitochondrial proteins with the nitrosothiols formation and nitrate production also changes towards NO3-, which shows the activation of oxida­tion reactions in heart mitochondria after NG administration. Accordingly, dose-dependent increase in lipid peroxidation (LP) products is shown, the hallmark of the membrane damage in mitochondria. It is established that the cause of oxidative stress, besides the dose-dependent increase in ROS produc­tion (hydroperoxide, superoxide and hydroxyl-radical), lies in the increase of free iron content, derived from the oxidation of mitochondrial iron-containing proteins. The iron interaction with hydroperoxide following Fenton reaction as well as free-radical decomposition of peroxynitrite, derived from NO3- are the possible cause of manifold increase in ROS as well as LP production, and RNS oxidation to NO3-. Thus, NO-dependent MPTP blockage, due to NO synthesis in mitochondria in vivo, results in the activation of both constituents of NO-cycle: NOS-dependent, due to Ca2+-dependent activation of mito-chondrial NOS, and nitrate-reductase-dependent, due to the increase in NO3- formation. However, increase in ROS pro­duction, augmented by the iron release, leads to the oxidative stress and the shift of RNS metabolism towards NO3- forma­tion, in spite of the activation of nitrate-reductase-dependent pathway of NO-cycle. It is shown that reversible MPTP open­ing in vitro diminishes ROS production, whereas MPTP block­age by cyclosporine A restores the ROS formation to control level. Thus, MPTP-dependent inhibition of ROS overproduc­tion both in vitro and in vivo, shows the importance of MPTP in the regulation of ROS and RNS metabolism in mitochon­dria.