Increasing evidence suggests that the dopamine transporter is situated almost exclusively on dopamine neurons. Accordingly, it is an valuable marker for Parkinson's disease and other pathological states of dopamine neurons. We previously demonstrated that the potent dopamine transport inhibitor [ 125 I]altropane (IACFT:E-Niodoallyl-2โค-carbomethoxy-3โค-(4-fluorophenyl)tropane) is a high affinity selective probe for the dopamine transporter in monkey brain and an effective SPECT imaging agent in nonhuman primate brain. We now report the binding properties of [ 125 I]altropane in postmortem tissue of normal human brain and compare the findings to Parkinson's diseased brain. In homogenates of human brain putamen, [ 125 I]altropane bound with high affinity (K D : 4.96 ฯฎ 0.38 nM, n ฯญ 4) and site density (B MAX : 212 ฯฎ 41.1 pmol/g original wet tissue weight) well within the density range reported previously for the dopamine transporter in this brain region. Drugs inhibited [ 125 I]altropane binding with a rank order of potency that corresponded closely to their rank order for blocking dopamine transport (r 0.98, P ฯฝ 0.001). In postmortem Parkinson's diseased brain, bound [ 125 I]altropane (1 nM) was markedly reduced (89%, 99% in putamen, depending on measures of nonspecific binding) compared with normal aged-matched controls (normal putamen: 49.2 ฯฎ 8.1 pmol/g; Parkinson's diseased putamen: 0.48 ฯฎ 0.33 pmol/g; n ฯญ 4). In vitro autoradiography, conducted in tissue sections at a single plane of the basal ganglia, revealed high levels of [ 125 I]altropane binding the caudate nucleus and putamen, but lower levels (73% of the caudate-putamen) in the nucleus accumbens (n ฯญ 7). In Parkinson's diseased brains (n ฯญ 4), [ 125 I]altropane binding was 13% of the levels detected in normal putamen, 17% of normal values in the caudate nucleus, and 25% of normal levels in nucleus accumbens. The association of [ 125 I]altropane to the dopamine transporter in human postmortem tissue, the marked reduction of [ 125 I]altropane binding in Parkinson's diseased brains, its rapid entry into brain and highly localized distribution in dopamine-rich brain regions, support its use as a probe for monitoring the dopamine transporter in vitro and in vivo by SPECT imaging.