In the present study, we have examined the expression and distribution of the metabolic marker neuron-specific enolase (NSE) in solid-tissue transplants of fetal substantia nigra (SN) to the striatum of intact and 6-hydroxydopamine lesioned mature rats. Immunocytochemistry was applied to label NSE and tyrosine hydroxylase (TH) respectively. Cellular content of NSE is indicative of metabolic activity as well as synaptogenesis/maturation. Three months after implantation, the fetal grafts exhibited intensely TH-immunoreactive neurons, typically organized in elongated clusters, especially along the graft-host border and along blood vessels penetrating into the graft interior. Moderate to high metabolic activity as indicated by NSE immunoreactivity was observed in neuronal perikarya, principally in non-TH immunoreactive areas. In contrast to these immunohistochemical findings, in situ hybridization for TH mRNA, carried out exclusively on grafts into the intact striatum, demonstrated DA cell bodies both at the graft-host interface and, significantly, throughout the graft interior. The number of transcripts per cell, moreover, did not differ significantly in these two locations. We propose that conditions at the graft-host border promote tissue-specific regulation of nigral DA neurons, and that this regulation occurs post-transcriptionally. Thus, DA neurons relatively distant from the host parenchyma are underregulated, resulting in a higher level of metabolic activity and an increased turnover of TH in the grafted neurons.