Since the expression of genes for platelet-derived growth factor (PDGF)-A and PDGF β-receptor are reciprocally regulated in vascular wall cells after balloon injury, we have investigated the ability of specific vasoactive molecules or growth factors to reproduce the injury pattern of gene expression in cultured rat smooth muscle cells (SMCs) and assessed the effect of inactivating α-thrombin on injury-induced expression of PDGF-A mRNA by vascular wall cells in vivo. The molecules investigated, to which vascular SMCs may be locally exposed after mechanical injury, included vasoactive factors (α- and β-adrenergic agonists, serotonin, histamine, angiotensin II, and endothelin) and growth factors (PDGF-AA, PDGF-BB, basic fibroblast growth factor, insulin-like growth factor, epidermal growth factor, and α-thrombin). In cultured rat SMCs, only α-thrombin (0.1–100 nM), among these compounds, produced the pattern of transiently increased PDGF-A and decreased PDGF β-receptor mRNA. PDGF-B chain mRNA levels remained undetectable in these cultured SMCs. The dependence of these changes in gene expression on the proteolytic activity of α-thrombin was shown by the interruption of altered gene expression or DNA synthesis after incubating the cultured SMCs with covalently inactivated α-thrombin using d-Phe-Pro-Arg chloromethyl ketone, a synthetic direct active-site irreversible inhibitor of α-thrombin. Continuous intravenous infusion of this synthetic antithrombin into baboons for 6 hours (100 nmol/kg per minute maintaining constant plasma levels of 3.0±0.5 μg/ml) after inducing balloon-catheter arterial injury also prevented the threefold increase in expression of PDGF-A mRNA characteristically exhibited by untreated mechanically injured vessels. We conclude that α-thrombin initiates an injury pattern of PDGF ligand and receptor gene expression both in vitro and in vivo. The importance of α-thrombin in the development of vascular lesion formation induced by mechanical injury and the possible usefulness of interrupting α-thrombin activity in the prevention of restenosis remain to be determined.