Sonic hedgehog (Shh) is a secreted morphogen crucial for cell fate decision, cellular proliferation, and patterning during vertebrate development. The intracellular Shh signalling is transduced by Smoothened (Smo), a seven-transmembrane spanning protein that belongs to the G-protein coupled receptor family. Among four families of Galpha subunits, Galphai has been thought to be responsible for transducing Shh signalling, while several lines of evidence indicated that other signalling pathways may be involved. We found that the G12 family of heterotrimeric G proteins and the small GTPase RhoA are involved in Shh/Smo-mediated cellular responses, including stimulation of target gene promoter and inhibition of neurite outgrowth of neuroblastoma cells. We also found that the G12/RhoA pathway is responsible for Smo-induced nuclear import of GLI3 which is thought to transduce Shh signals to nucleus. Furthermore, misexpression of a G12-specific GTPase-activating protein in rat neural tubes leads to pertubation of motor neurone and interneurone development, mimicking the effects of decreased Shh signalling. These results show that Shh signalling is mediated in part by activating G12 family coupled signalling pathways. The participation of RhoA, a pivotal molecular switch in many signal transduction pathways, may help explain how Shh can trigger a variety of cellular responses.