Exposure of rat aortic vascular smooth muscle cells to alpha-thrombin resulted in the appearance of sis-inducing factor-A (SIF-A)-like DNA binding activity. This response to alpha-thrombin was delayed (detectable at 1 hour) compared with the rapid activation (15 to 30 minutes) by platelet-derived growth factor and the cytokine interleukin-6. Alpha-Thrombin-induced SIF-A was sensitive to treatment with the tyrosine kinase inhibitor genistein. The thrombin inhibitor hirudin prevented the alpha-thrombin-mediated SIF-A induction. Cycloheximide had no effect on the ability of alpha-thrombin to induce SIF-A, suggesting that induction does not require new protein synthesis. Alpha-Thrombin-induced SIF-A could be resolved into two additional subcomplexes termed SIF-AFand SIF-AS. Antibodies against Stat3 reacted with alpha-thrombin-induced SIF-AF, suggesting that Stat3 or a related protein is present in this subcomplex. Induction of SIF-A DNA binding activity may contribute to alpha-thrombin-mediated cellular responses, including wound healing, cell proliferation, and inflammation in the vasculature. (Hypertension. 1997;29[part 2]:356-360.)