Thrombin, a multifunctional protein, has been found to be involved in cellular mitogenesis, tumor growth, and metastasis, in addition to its well known effects on the initiation of platelet aggregation and secretion and the conversion of fibrinogen to fibrin to form blood clots. These properties of thrombin rely on its action as a serine protease, which cleaves the N-terminal region of a 7-transmembrane G protein receptor (protease-activated receptor, PAR-1), thus exposing a tethered end hexapeptide sequence capable of activating its receptor. Little is known about its effect on genes that regulate the cell cycle. This study was undertaken to investigate the possible mechanisms by which thrombin regulates tumor cell growth in several tumor cell lines: human CHRF megakaryocyte, DU145 prostate, MDAMB231 and MCF7 breast, U3A fibrosarcoma, and 2 murine fibroblast cell lines, MEFp53(-/-) and CD STAT(-/-). We have found that thrombin under the conditions of culture employed inhibits cell growth by both up-regulation of p21(waf/cip1) and induction of caspases via its PAR-1 receptor. The increased expression of p21(waf/cip1) by thrombin was p53 independent, STAT1 dependent, and protein synthesis independent. This was associated with tyrosine phosphorylation of JAK2 and STAT1, and nuclear translocation of STAT1. Induction of apoptosis is also PAR-1-specific, STAT1-dependent, and associated with up-regulation of caspases 1, 2, and 3. Our study establishes, for the first time, a link between PAR-1 receptor activation with the STAT signal pathway, which leads to cell cycle control and apoptosis. This observation broadens our understanding of the mechanism of PAR-1 activation and its effect on cell growth, and could possibly lead to therapeutic approaches for the treatment of cancer.