To investigate the topography of morphine distribution in the human brain, a method has been developed to detect morphine immunohistochemically. In this study hippocampus tissue from victims of heroin overdose (blood morphine concentrations 220 ng/g-1500 ng/g; 6-MAM positive urine sample), known for its high concentration of mu-opiate receptors was used. The immunohistochemical staining was performed with an anti-morphine antiserum originally developed for radio-immunoassays. In comparison with control specimens from cases of sudden death without morphine exposition or a history of heroin abuse, the brains from victims of heroin overdose showed selectively stained ganglion cells, axons and dendrites, suggesting a massive concentration of morphine in the neuronal structures.