Nigrostriatal neurons expressing RET protein, a receptor protein tyrosine kinase of glial cell line-derived neurotrophic factor (GDNF) were investigated in rats using retrograde neural tracing with horseradish peroxidase (HRP) combined with immunohistochemistry. HR/RET double-labeled neurons were abundantly distributed in the substantia nigra pars compacta ipsilateral to the caudate-putamen stereotaxically injected with HRP. Almost all the HRP-labeled neurons in nigra exhibited RET-like immunoreactivity, which however constituted more than half of the RET-immunoreactive cells. Our results present morphological evidence that GDNF-RET interaction plays important roles in physiological processes of nigrostriatal neuronal circuits of mammals.