Much reduced thiol appears in the cerebral cortex during intensive stimulation of peripheral nerves. This affects specific chemical receptors of mitochondria causing alterations in their structure and physicochemical properties. An increased content of reduced thiol compounds may lead to mitochondrial swelling. The visual estimation of histochemically demonstrated activity of succinic dehydrogenase and diphosphopyridine nucleotide (DPN) diaphorase shows the picture of an increase in the intramitochondrial formazan deposits and their intensified coloration. Mitochondrial swelling causes impairment in the respiratory cycle. It should be noted that in this process the activity of cytochrome oxidase is almost completely inhibited, the system of electron transfer is impaired (DPN diaphorase) and the activity of succinic dehydrogenase shows a considerable drop. Alterations of respiration are reversible, and after the cessation of stimulation (20 minutes for DPN diaphorase and succinic dehydrogenase and 50 minutes for cytochrome oxidase) the activity of the enzymes returns to normal. Histochemical alterations undergo an opposite development. Mitochondrial changes detected by histochemical tests with tetrazolium salts (dimethylthiazole tetrazolium (MTT) and nitro-blue tetrazolium) reflected extremely subtle alterations in the functional state of these organelles, although they fail to serve as criteria for the quantitative estimation of the dehydrogenase and diaphorase activity. In these experiments mitochondrial swelling was accompanied by a drop in the enzymic activity of this organelle. Also described was the temporal dissociation of the change in chemical and electrical activity of the cerebral cortex.