Receptor modifications in human postmortem material were studied by quantitative autoradiography. Alterations of several neurotransmitter receptors in neurodegenerative diseases such as senile dementia and Huntington's chorea, in lesions of specific brain pathways, like the visual pathway or after drug treatments, were examined. In all these situations alterions of the density or localization of receptors were seen using autoradiography. The results suggest that several mechanisms of receptor adaptation operate in the human brain. These mechanisms include: compensatory changes in receptor density as a consequence of cell loss, in some cases preceding the neuropathological changes; differential alterations in receptors depending on their location in a given pathway, for example in the visual pathway or selective homologous or heterologous modification of receptors after drug treatment.