Measurement of the frontal cortex and the medulla oblongata pH at autopsy revealed that those brains from individuals who died after long terminal phase had lower pHs than those who died quickly. These low pHs (pH 6.0-6.5) corresponded to lactic acid concentrations (20-25 mM) which are known to be neurotoxic. The cerebrospinal fluid pH and blood pH correlated with the cortical pH. The significance of these observations to the study of the biochemistry and histology on post-mortem human brain tissue and their possible clinical relevance are discussed.