Post-mortem changes in the activity of various cerebral dehydrogenases have been investigated in 41 human brains. Histoenzymatic examinations were conducted 1, 4, 6, 12, 18 and 24 h after clinically established death. The results have shown that the individual dehydrogenase become inactivated by the post-mortem processes at different rates. Succinate and lactate dehydrogenase activities were lost at first rate, followed by glucose-6-phosphate and beta-hydroxybutyrate dehydrogenase and then by isocitrate and NADH and NADPH dependent reductases, alpha-glycerophosphate dehydrogenase remained active for the longest period after death. The structural sequence of the post mortem loss of respiratory enzymes in the human brain was opposite to that at which these enzymes appeared during ontogenic development. The topography of the post mortem inactivation of cerebral dehydrogenases instead was adverse to the order of constitution of the individual parts of the CNS in the course of phylogenetic development.