Regulator of G-protein signaling (RGS) domains bind directly to GTP-bound Gα subunits and accelerate their intrinsic GTPase activity by up to several thousandfold. The selectivity of RGS proteins for individual Gα subunits has been illustrated. Thus, the expression of RGS proteins can be used to inhibit signaling pathways activated by specific G protein-coupled receptors (GPCRs). This article describes the use of specific RGS domain constructs to discriminate among G i/o, G q-and G 12/13-mediated activation of phospholipase C (PLC) isozymes in COS-7 cells. Overexpression of the N terminus of GRK2 (amino acids 45-178) or p115 RhoGEF (amino acids 1-240) elicited selective inhibition of Gα q- or Gα 12/13-mediated signaling to PLC activation, respectively. In contrast, RGS2 overexpression was found to inhibit PLC activation by both G i/o- and G q-coupled GPCRs. RGS4 exhibited dramatic receptor selectivity in its inhibitory actions; of the G i/o- and G q-coupled GPCRs tested (LPA 1, LPA 2, P2Y 1, S1P 3), only the G q-coupled lysophosphatidic acid-activated LPA 2 receptor was found to be inhibited by RGS4 overexpression.