M1 muscarinic cholinergic receptors, G1 and G11 (Gq/11), and phospholipase C-beta 1 were highly purified from both natural sources and cells that express the appropriate cDNA's. When the proteins were co-reconstituted into phospholipid vesicles, the receptor efficiently and selectively promoted the activation of Gq/11, leading to marked stimulation of PLC activity in the presence of GTP gamma S. No stimulation was observed in the presence of GTP, however, which led to the finding that PLC-beta 1 stimulates the hydrolysis of GQ/11-bound GTP at least 50-fold. Thus, PLC-beta 1 is a GTPase activating protein, a GAP, for its physiologic regulator Gq/11. We discuss the implications of PLC-beta 1's GAP activity on the M1 muscarinic cholinergic signaling pathway.