The dopamine transporter is the primary means of inactivating synaptic dopamine as well as a major site of action for psychostimulants (such as cocaine and amphetamine) and for neurotoxins that induce parkinsonism. In the present study, a human dopamine transporter partial cDNA clone obtained by polymerase chain reaction exhibited 87% and 89% identity at the nucleic acid and amino acid levels, respectively, with transmembrane domains 3-5 of the rat homolog. This clone was used to quantitate human dopamine transporter mRNA by nuclease protection assay. The postmortem content of dopamine transporter mRNA in the substantia nigrae of 18- to 57-yr-old subjects was relatively constant, while in subjects greater than 57 yr old, a precipitous (greater than 95%) decline in substantia nigra dopamine transporter mRNA was evident. In contrast, tyrosine hydroxylase mRNA in the same samples declined in a linear manner with increasing age. In situ hybridization experiments confirmed the profound loss of dopamine transporter gene expression in melanin-positive (presumptive dopamine) nigral neurons. These data may begin to shed light on compensatory changes occurring in human dopamine neurons during normal aging.