It is currently believed that reactive forms of oxygen, produced by the mitochondrial electron transportation chain, play an important part in the process of senescence of the central nervous system and in the pathogeny of Alzheimer-type dementia. Recent studies have shown that mutations in mitochondrial genes for cytochrome-C-oxidase for two subunits are a risk factor for the development of Alzheimer's disease in the elderly. Damage to this enzyme increases the mitochondria's production of reactive forms of oxygen which play an important part in the process of accumulation of beta-amyloid deposits in the CNS. Administration of antioxidants (alpha-tokopherol and/or selegiline) cheeks the progression of Alzheimer's disease in moderately-advanced stages. This confirms the crucial role of reactive forms of oxygen in the pathogeny of dementive syndromes.