GABA is one of the most important inhibitory neurotransmitters in the substantia nigra. Functions of GABA are mediated by two major types of GABA receptors, namely the GABA(A) and GABA(B) receptors. Subunits of both the GABA(A) and GABA(B) receptors have been cloned and functional characteristics of the receptors depend on their subunit compositions. In order to characterize the cellular localization of GABA(B)R1 and GABA(A)alpha1 subunit immunoreactivity in subpopulations of neurons in the rat substantia nigra, double and triple immunofluorescence was employed. Over 90% of tyrosine hydroxylase-immunoreactive neurons in the substantia nigra pars compacta were found to display immunoreactivity for GABA(B)R1. In contrast, immunoreactivity for GABA(A)alpha1 was found to be primarily displayed by neurons in the substantia nigra pars reticulata. Around 85% of the GABA(A)alpha1-immunoreactive reticulata neurons were found to display parvalbumin immunoreactivity and some GABA(A)alpha1-positive reticulata neurons were found to be parvalbumin negative. In addition, triple-labeling experiments revealed that at the single cell level, the tyrosine hydroxylase-positive, i.e. the dopaminergic neurons in the compacta displayed intense immunoreactivity for GABA(B)R1 but not GABA(A)alpha1 receptors. The parvalbumin-positive neurons in the reticulata displayed intense immunoreactivity for GABA(A)alpha1 but not GABA(B)R1 receptors. The present results demonstrate in the same sections that there is a distinct pattern of localization of GABA(B)R1 and GABA(A)alpha1 receptor immunoreactivity in different subpopulations of the rat substantia nigra and provide anatomical evidence for GABA neurotransmission in the subpopulations of nigral neurons.