The D3 dopamine receptor, belonging to the D2-like receptor subfamily, displays both specific pharmacology and a unique pattern of expression, especially compared to the D1 and D2 receptors. A rat D3 complementary RNA probe labelled with [35S]UTP was used to map the neurons expressing the D3 receptor gene in the rat brain. In particular, we identified the phenotype of the neurons containing D3 messenger. RNA in the nucleus accumbens, with respect to the striatal segregated populations of enkephalin and substance P neurons, by using strategies of double in situ hybridization. In addition, comparison with D1 and D2 receptor gene expression was performed to study the potential co-localization of several dopamine receptors within the same neurons in the ventral striatum. The highest level of D3 messenger RNA was detected in the islands of Calleja and the nucleus accumbens. Other areas such as the caudate-putamen, some hypothalamic, thalamic and cortical areas and lobule 10 of the cerebellum also contained low but significant levels of D3 messenger RNA. Our results demonstrate that the D3 receptor is co-expressed either with D1 or with D2 receptor, both in the core and shell regions, in a subpopulation of substance P and enkephalin neurons, respectively. They also suggest that a significant part of the accumbal neurons only express either D1 or D2, without co-expression with the D3 receptor. These data imply that dopamine in the nucleus accumbens may act on each population of efferent neurons via the co-expression of two distinct dopamine receptors (D1+D3 or D2+D3), and that synergistic D1-like/D2-like effects may occur at the level of a single neuron, through the co-expression of D1 and D3 receptors in a significant proportion of substance P neurons (16% or 26% in the core and shell regions, respectively.