In an experimental model, it was shown that repetitive periods of hypoxia/reoxygenation (H/R) [5 cycles of 5 min hypoxia (12% O(2) in N(2) ) followed by 15 min normoxia, daily for three weeks] attenuated basal and stimulated in vitro lipid peroxidation, as well as H(2)O(2) production in liver and brain mitochondria of rats exposed to acute severe hypoxia. Adaptation to moderate H/R enhanced in mitochondria the production and activity of reactive oxygen species scavengers, such as glutathione, manganese superoxide dismutase, glutathione peroxidase, and glutathione-S-transferase. It was demonstrated that the maintenance of GSH-redox cycle by activation of glutathione reductase and NADP(+) -dependent isocitrate dehydrogenase is an integral part of the biochemical adaptive mechanism of oxidative tolerance to new damaging factor. Brain mitochondria showed more sensitivity to oxidative stress than liver mitochondria, and long-lasting sessions of H/R affect differentially their pro-/antioxidant homeostasis.