The chemical changes documented in corpses include change in the hydrogen ion exponent and redox potential, the formation of endogenous ethyl alcohol often accompanied by methanol, and the occurrence of various haemoglobin derivatives. Also, the possibility of the formation of cyanide ions in postmortem blood is particularly important in interpreting the result of analyses for cyanide content. An investigation on the breakdown of the endogenous substance important for toxicological studies included the determination of acetylcholinesterase activity in the blood, brain, and liver and also its association with the concentrations of haemoglobin. The activity of this enzyme determined in the blood did not undergo changes for at least 60 days when stored in a refrigerator. Similar results were obtained for brain tissues. The activity of the enzyme decreased successively when stored at room temperature. Postmortem degradation not only concerns volatile compounds but also many other substances, such as benzodiazepines, tricyclic antidepressants, and phenothiazines. Many nonvolatile compounds, drugs among them, could be metabolized by micro-organisms during the putrefactive process. Those who conduct postmortem examinations of bodies in order to determine cause of death must be familiar with the various chemical changes in the decomposition of bodies, so as not to assume that the presence or levels of various chemicals found postmortem existed in the body just prior to death. 14 figures and 48 references