The characteristics of [3H]spiperone binding in human brain were compared in three areas of the nigrostriatal pathway: areas containing the nerve terminals (caudate nucleus, putamen and pallidum); the area containing the cell bodies (substantia nigra pars compacta); the area containing the dendrites (pars reticulata). The affinity constants were calculated from saturation binding curves and from the kinetics of association and dissociation. The pharmacological profiles of the receptors was also established by displacement studies. The affinity constants and pharmacological profiles were similar in the striatum and the substantia nigra, although the latter contained a much smaller number of sites. In the substantia nigra, however, curved Scatchard plots were obtained, indicating that a second lower affinity site binding [3H]spiperone was also present. A considerable proportion (50%) of [3H]spiperone binding to nigral membranes could be displaced by the serotonin antagonist cinanserine , compared to the striatum (20%). The effect of post-mortem conditions on binding levels was studied in the rat. A loss of 20% occurred during the first hours after death, but was stable by 6 h until at least 24 h.