Signaling pathways that link extracellular factors to activation of the monomeric guanosine triphosphatase (GTPase) Rho control cytoskeletal rearrangements and cell growth. Heterotrimeric guanine nucleotide–binding proteins (G proteins) participate in several of these pathways, although their mechanisms are unclear. The GTPase activities of two G protein α subunits, Gα₁₂ and Gα₁₃, are stimulated by the Rho guanine nucleotide exchange factor p115 RhoGEF. Activated Gα₁₃ bound tightly to p115 RhoGEF and stimulated its capacity to catalyze nucleotide exchange on Rho. In contrast, activated Gα₁₂ inhibited stimulation by Gα₁₃. Thus, p115 RhoGEF can directly link heterotrimeric G protein α subunits to regulation of Rho.