The subject of the study is a fatal poisoning with levomepromazine. A toxicological investigation carried out by means of the liquid chromatographic method with mass detection (LC/MS) revealed levomepromazine and its 24 metabolites in postmortem specimens in various concentrations depending on the specimen tested. The study was undertaken not only for judicial evidence, but also to investigate whether any changes occur in the concentrations of xenobiotics found in blood during the postmortem interval. The investigation confirmed such differences in xenobiotic concentrations, proving tha~the passive diffusion of xenobiotics in a body takes place. RESUMECette etude presente un cas d'intoxication fatale SI la levomepromazine;les analyses toxicologiques effectuees par chromatographie Iiquide-spectrometrie de masse (CUSM) ont permis de demontrer la presence de levomepromazine et 24 de ses metabolites dans plusieurs prelevements biologiques postmortem SI diverses concentrations. Cette etude a ete faite autant pour recueillir certains elements de preuve que pour etudier les changements dans les concentrations sanguines de certains xenobiotiques durant la perlode postmortem. Le resultat de cette etude nous a permis de confirmer ces differences dans les concentrations sanguines postmortem prouvant ainsi leur diffusion passive dans I'organisme.