Platelets respond to various stimuli with rapid changes in shape followed by aggregation and secretion of their granule contents. Platelets lacking the α-subunit of the heterotrimeric G protein G_q do not aggregate and degranulate but still undergo shape change after activation through thromboxane-A₂ (TXA₂) or thrombin receptors. In contrast to thrombin, the TXA₂ mimetic U46619 led to the selective activation of G₁₂ and G₁₃ in Gα_q-deficient platelets indicating that these G proteins mediate TXA₂ receptor-induced shape change. TXA₂ receptor-mediated activation of G₁₂/G₁₃ resulted in tyrosine phosphorylation of pp72^syk and stimulation of pp60^c-src as well as in phosphorylation of myosin light chain (MLC) in Gα_q-deficient platelets. Both MLC phosphorylation and shape change induced through G₁₂/G₁₃ in the absence of Gα_q were inhibited by the C3 exoenzyme from Clostridium botulinum, by the Rho-kinase inhibitor Y-27632 and by cAMP-analogue Sp-5,6-DCl-cBIMPS. These data indicate that G₁₂/G₁₃ couple receptors to tyrosine kinases as well as to the Rho/Rho-kinase–mediated regulation of MLC phosphorylation. We provide evidence that G₁₂/G₁₃-mediated Rho/Rho-kinase–dependent regulation of MLC phosphorylation participates in receptor-induced platelet shape change.