Proliferation of cerebellar granular neuronal precursors (CGNPs) is mediated by Sonic Hedgehog (Shh), which activates the Patched and Smoothened (Smo) receptor complex. Although its protein sequence suggests that Smo is a G protein coupled receptor (GPCR), the evidence that this receptor utilizes heterotrimeric G proteins as downstream effectors is controversial. In Drosophila, Gα(i) is required for Hedgehog (Hh) activity, but the involvement of heterotrimeric G proteins in vertebrate Shh signaling has not yet been established. Here, we show that Shh-induced proliferation of rat CGNPs is enhanced strongly by the expression of the active forms of Gα(i/o) proteins (Gα(i1), Gα(i2), Gα(i3), and Gα(o)) but not by members of another class (Gα(12)) of heterotrimeric G proteins. Additionally, the mRNAs of these different Gα(i) members display specific expression patterns in the developing cerebellum; only Gα(i2) and Gα(i3) are substantially expressed in the outer external granular layer, where CGNPs proliferate. Consistent with this, Shh-induced proliferation of CGNPs is reduced significantly by knockdowns of Gα(i2) and Gα(i3) but not by silencing of other members of the Gα(i/o) class. Finally, our results demonstrate that Gα(i2) and Gα(i3) locate to the primary cilium when expressed in CGNP cultures. In summary, we conclude that the proliferative effects of Shh on CGNPs are mediated by the combined activity of Gα(i2) and Gα(i3) proteins.