G protein–coupled receptors (GPCRs) transmit signals by forming active-state complexes with heterotrimeric G proteins. It has been suggested that some GPCRs also assemble with G proteins before ligand-induced activation and that inactive-state preassembly facilitates rapid and specific G protein activation. However, no mechanism of preassembly has been described, and no functional consequences of preassembly have been demonstrated. Here we show that M₃ muscarinic acetylcholine receptors (M3R) form inactive-state complexes with G_q heterotrimers in intact cells. The M3R C terminus is sufficient, and a six-amino-acid polybasic sequence distal to helix 8 (⁵⁶⁵KKKRRK⁵⁷⁰) is necessary for preassembly with G_q. Replacing this sequence with six alanine residues prevents preassembly, slows the rate of G_q activation and decreases steady-state agonist sensitivity. That other G_q-coupled receptors possess similar polybasic regions and also preassemble with G_q suggests that these GPCRs may use a common preassembly mechanism to facilitate activation of G_q heterotrimers.