Local accumulation of plasminogen activator inhibitor-1 (PAI-1) in response to thrombosis has been implicated not only in inhibition of fibrinolysis but also in the pathogenesis of vascular disease. To determine whether thrombin, known to be released from thrombi, can induce expression of PAI-1 in vascular smooth muscle, bovine aortic smooth muscle cells were exposed to highly purified bovine thrombin. Thrombin, in the absence of serum, induced production of PAI-1 by bovine aortic smooth muscle cells in a dose-dependent manner. PAI-1 activity in the conditioned media reached a maximum with 12 nM thrombin. Metabolic labeling with [35S]methionine demonstrated that the elaborated PAI-1 was newly synthesized and that it comprised both a cleaved inactive 42-kd form and an uncleaved active 46-kd form. The increase of PAI-1 activity in the media paralleled the thrombin-induced increase in the concentration of the 46-kd form. Preincubation of thrombin with hirudin, a specific inhibitor of thrombin, or with d-phenylalanyl-l-prolyl-l-arginine chloromethyl ketone, an inhibitor of the active site of thrombin, prevented the induction of PAI-1 synthesis. The stimulatory effect of thrombin on PAI-1 synthesis was also evident at the level of expression of mRNA, with steady-state PAI-1 mRNA levels increasing by 100% in 4–8 hours. When the bovine aortic smooth muscle cells were exposed to transforming growth factor-βl, an agonist shown previously to increase PAI-1 synthesis in diverse cell types, synergy with thrombin was evident. Thus, production of PAI-1 by vascular smooth muscle cells is augmented by thrombin, potentially predisposing the cells to persistent thrombi and to vasculopathy at sites of thrombosis and at sites of endothelial injury or denudation.