Dopamine D3 receptors (Sokoloff et al., 1990) have been shown to be related to dopamine D2 receptors and have been suggested to play a role in mediating the antipsychotic effects of neuroleptics. So far studies on the expression of D3 mRNA and of binding sites with pharmacological characteristics of D3 receptors have been restricted to rat brain. Using in situ hybridization histochemistry, we demonstrate that D3 mRNAs are enriched in human n, accumbens and in the islands of Calleja. In addition, D3 mRNA was detected at very low levels in anterior caudate and putamen with a rostro-caudally decreasing gradient and in hypothalamic mammillary nuclei. In receptor autoradiographic binding studies, the islands of Calleja were found to be labeled by [125I]iodosulpride and [3H]CV 205 502 but not by [3H]raclopride and [3H]YM 09151-2. Pharmacological analysis of binding of the D2/D3 ligand [3H]CV 205 502 in n. accumbens and caudate-putamen is consistent with the presence of D3 receptor sites in ventral striatum. Overall distribution and pharmacology of D3 sites in human and rat brain appear to be similar. Presence and distribution of D3 receptors in human brain are compatible with the notion that D3 receptors might be involved in mediating the clinical effects of antipsychotics.