The present experimental investigations were inspired by the necessity of standardizing the procedures and analytical methods employed in hair analysis aiming at a retrospective evaluation of ingestion of various xenobiotics. Thus, in keeping with the principal premises, the main objective of the study was development of unique, novel chemico-toxicological procedures for analyzing hair content of psychoactive substances in two basic groups of substances of abuse: opioids (morphine, 6-monoacetylmorphine, codeine) and amphetamines (amphetamine, methamphetamine, MDA, MDMA, MDEA) by HPLC-APCI-MS-MS, followed by verification of the thus worked out procedures in medico-legal practice through opinionating in selected group of patients deceased due to fatal psychoactive substance poisoning (cause of death determination). Determinations of opioids and amphetamines in the hair biological matrix were performed using high performance liquid chromatography - atmospheric pressure chemical ionization - tandem mass spectrometry (HPLC-APCI-MS-MS). In the group of fatal poisonings by ,,Polish heroine", hair segmental analysis confirmed the abuse profile of the opiate or mixed (opiate-amphetamine) type, which to some measure is characteristic of Polish drug addiction, indicating the presence of these xenobiotics in the investigating hair samples in the premortem period.