A functional gamma-aminobutyric acid (GABA) B receptor is the first metabotropic receptor known to be composed of two heteromeric subunits, GABA B R1 and GABA B R2. Our previous report [Neuroscience 99 (2000) 65] has demonstrated that subpopulations of neurons in the rat substantia nigra display distinct patterns of distribution of GABA B R1 receptor immunoreactivity. A robust level of GABA B R1 receptor is only found in the dopaminergic neurons of the substantia nigra pars compacta (SNc). The objective of the present study was to determine the precise cellular localization of GABA B R2 subunit in the rat substantia nigra using double immunofluorescence. Neuropilar elements in the SNc and the substantia nigra pars reticulata (SNr) were found to display GABA B R2 immunoreactivity. In addition, the tyrosine hydroxylase-immunoreactive dopaminergic neurons and the parvalbumin-immunoreactive GABAergic neurons in the SNr were also found to display GABA B R2 immunoreactivity. The present results thus demonstrate that a functional GABA B receptor may be expressed by the dopaminergic neurons in the SNc. It is less clear whether neurons in the SNr express a functional GABA B receptor. The present findings have important functional implications in GABA neurotransmission in the substantia nigra.