The objective of the present study was to improve forensic medical diagnostics of the cases of death associated with morphine poisoning based on the investigation into the biochemical changes in blood and pericardial fluid as well as morphological changes in the myocardial structures. The studies were carried out with the use of thin-layer chromatography, colorimetric and morphological methods including hematoxylin and eosin, Lee's methylene blue, and van Gieson's picrofuscin staining. These techniques were supplemented by light and polarization microscopy. The study has demonstrated the presence of morphine in 99.16% of the blood and pericardial samples obtained in the cases of poisoning. The comparison of the results of biochemical and pathomorphological studies of the myocardium made it possible to evaluate the functional and morphological conditions of the heart in the case of acute morphine poisoning during the period of chronic drug intoxication.