5HT1A serotonin (5-hydroxytryptamine) receptors have been characterized by ligand binding in a recombinant Chinese Hamster Ovary cell line expressing the human receptor gene. The agonist ligand [3H]2-(N,N-dipropylamino)-8-hydroxy-1,2,3,4-tetrahydronaphthalene ([3H]8-OH-DPAT) and the antagonist [3H]spiperone were used. For both radioligands the binding sites labelled have the properties of 5HT1A receptors and most antagonists show roughly equal affinities for the receptors labelled by either [3H]8-OH-DPAT or [3H]spiperone. Agonists, however, show higher affinities for the sites labelled by [3H]8-OH-DPAT and the antagonist spiperone conversely shows a higher affinity for the sites labelled by [3H]spiperone. Whereas [3H]8-OH-DPAT binding is inhibited by guanosine triphosphate (GTP) the binding of [3H]spiperone is increased by GTP. A model is proposed for the results whereby [3H]8-OH-DPAT labels a form of the receptor coupled to a G-protein and [3H]spiperone labels a form of the receptor uncoupled from G-proteins (or possibly coupled to a different G-protein).