Platelet activation due to vascular injury is essential for hemostatic plug formation, and is mediated by agonists, such as thrombin, which trigger distinct receptor-coupled signaling pathways. Thrombin is a coagulation protease, which activates G protein-coupled protease-activated receptors (PARs) on the surface of platelets. We found that C57BL/6J and BALB/C mice that are deficient in protein kinase C theta (PKC theta), exhibit an impaired hemostasis, and prolonged bleeding following vascular injury. In addition, murine platelets deficient in PKC theta displayed an impaired thrombin-induced platelet activation and aggregation response. Lack of PKC theta also resulted in impaired a-granule secretion, as demonstrated by the low surface expression of CD62P, in thrombin-stimulated platelets. Since PAR4 is the only mouse PAR receptor that delivers thrombin-induced activation signals in platelets, our results suggest that PKC theta is a critical effector molecule in the PAR4-linked signaling pathways and in the regulation of normal hemostasis in mice. (C) 2009 Elsevier Inc. All rights reserved.