Although the oxytocin receptor modulates intracellular Ca2+ ion levels in myometrium, the identities of signal molecules have not been clearly clarified. Our previous studies on oxytocin receptor signalling demonstrated that 80 kDa Ghα is a signal mediator [Baek, Kwon, Lee, Kim, Muralidhar and Im (1996) Biochem. J. 315, 739–744]. To elucidate the effector in the oxytocin receptor signalling pathway, we evaluated the oxytocin-mediated activation of phospholipase C (PLC) by using solubilized membranes from human myometrium and a three-component preparation containing the oxytocin receptor–Ghα–PLC-δ1 complex. PLC-δ1 activity in the three-component preparation, as well as PLC activity in solubilized membranes, was increased by oxytocin in the presence of Ca2+ and activated Ghα (GTP-bound Ghα). Furthermore the stimulated PLC-δ1 activity resulting from activation of Ghα via the oxytocin receptor was significantly attenuated by the selective oxytocin antagonist desGly-NH2d(CH2)5[Tyr(Me)2,Thr4]ornithine vasotocin or GDP. Consistent with these observations, co-immunoprecipitation and co-immunoadsorption of PLC-δ1 in the three-component preparation by anti-Gh7α antibody resulted in the PLC-δ1 being tightly coupled to activated Ghα on stimulation of the oxytocin receptor. These results indicate that PLC-δ1 is the effector for Ghα-mediated oxytocin receptor signalling.