The hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP₂) by phospholipase C yields the second messengers inositol 1,4,5-trisphosphate (InsP₃) and 1,2-diacylglycerol. This activity is regulated by a variety of hormones through G protein pathways. However, the specific G protein or proteins involved has not been identified. The α subunit of a newly discovered pertussis toxin-insensitive G protein (G_q) has recently been isolated and is now shown to stimulate the activity of polyphosphoinositide-specific phospholipase C (PI-PLC) from bovine brain. Both the maximal activity and the affinity of PI-PLC for calcium ion were affected. These results identify G_q as a G protein that regulates PI-PLC.