One of the primary functions of the oxytocin receptor is to modulate intracellular calcium levels in myometrium. The oxytocin receptor has been purified and cloned. Although it has been suggested that oxytocin receptor couples with a GTP-binding regulatory protein (G-protein), the identity of this G-protein remains unclear. To elucidate the mechanism of oxytocin receptor signalling, we used the oxytocin–receptor–G-protein ternary complex preparation from human myometrium, and evaluated oxytocin-mediated activation of [ 35 S]guanosine 5´-[γ-thio]triphosphate ([ 35 S]GTP[S]) binding and [α- 32 P]GTP photoaffinity labelling to a G-protein. Binding of [ 35 S]GTP[S] and the intensity of the [α- 32 P]GTP photoaffinity labelled protein resulting from activation of the oxytocin receptor were significantly attenuated by the selective oxytocin antagonist, desGlyNH 2 d(CH 2 ) 5 [Tyr(Me) 2 ,Thr 4 ]OVT. Furthermore, the molecular mass of the specific GTP-binding protein was ~80 kDa; homologous with the G hα family, the new class of GTP-binding proteins first identified in rat liver that couples to the α 1B -adrenoceptor. Consistent with these observations, in co-immunoprecipitation and co-immunoadsorption of the oxytocin receptor in the ternary complex preparation by anti-G h7α antibody, the G hα family protein tightly coupled to the oxytocin receptor. These findings demonstrate that oxytocin receptor couples with ~80 kDa G hα in signal mediation.