Binding of dopamine receptor ligands to human D2 and D3 receptors was characterized in Chinese hamster ovary (CHO) cells using the dopamine D2 receptor antagonist [125I] iodosulpiride. Only limited binding selectivity was observed for known dopamine D2 receptor antagonists from a variety of chemical classes, which included haloperidol, chlorpromazine, sulpiride, pimozide and cis flupenthixol. The most selective compound from this group were (+)butaclamol and domperidone which showed 5-fold D3 selectivity. A number of high affinity dopamine receptor agonists, including apomorphine and bromocriptine, also failed to demonstrate selectivity. In contrast, the natural ligand dopamine and the efficacious synthetic agonists quinpirole, (+)4-propyl-9-hydroxynapthoxazine (PHNO), 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene (6,7-ADTN), 7-OH DPAT and N-0434 showed marked apparent human dopamine D3 (hD3) receptor selectivity. In the aminotetralin series, this selectivity was observed preferentially with analogs of the 6,7-rotamer compared with compounds from the 5,6-rotamer series. Functional coupling of the hD3 receptor was investigated in a number of cell lines in which the hD3 receptor was stably expressed, including CHO cells, the neuroblastoma-glioma hybrid cell line NG108-15 and a rat 1 fibroblast cell line. There was no evidence of functional coupling of the hD3 receptor to adenylate cyclase, arachidonic acid release, phospholipase C activation, K+ currents or calcium mobilization in any of the cell lines examined. Furthermore, guanine nucleotides failed to inhibit the binding of [3H] N-0437 to hD3 receptors in any of the three cell lines. There may be a number of explanations for these results. These cell lines may not have the appropriate G-protein or secondary messenger systems that are coupled to the hD3 receptor in situ. Alternatively, this receptor may couple by a mechanism that is as yet undefined. The finding that a wide range of structurally diverse human dopamine D2 (hD2) receptor agonists have an apparent hD3 selectivity may imply that the hD3 receptor exists predominantly in a high affinity state.