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-A 2 (TXA 2) or thrombin receptors. In contrast to thrombin, the TXA 2 mimetic U46619 led to the selective activation of G 12 and G 13 in Gα q-deficient platelets indicating that these G proteins mediate TXA 2 receptor-induced shape change. TXA 2 receptor-mediated activation of G 12/G 13 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 G 12/G 13 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 G 12/G 13 couple receptors to tyrosine kinases as well as to the Rho/Rho-kinase–mediated regulation of MLC phosphorylation. We provide evidence that G 12/G 13-mediated Rho/Rho-kinase–dependent regulation of MLC phosphorylation participates in receptor-induced platelet shape change.