Brain tissues of rats, mice and humans taken during autopsy were incubated in the medium containing 3,3'-diaminobenzidine (DAB), permitting to recognize mitochondrial cytochromes. The product of DAB oxidation was located in mitochondria and lypofuscin granules of the studied objects. Besides, the reaction product was found in the rat's brain structures of intermediate type, bearing resemblance to both mitochondria and lypofusion granules. These structures are regarded as a transitional phase of mitochondria transformation into lypofuscin. Cytochemical investigation of the brain of mice and man in long-afterdeath-terms demonstrated that a postmortal degradation of mitochondria does not result in lypofuscin formation.