The airway remodeling that occurs in asthma is characterized by an excess of extracellular matrix (ECM) deposition in the submucosa, hyperplasia/hypertrophy of smooth muscle, goblet cell metaplasia, and accumulation of fibroblasts/myofibroblasts. The urokinase-type plasminogen activator (uPA)/plasmin system participates in pericellular proteolysis and is capable of directly degrading matrix components, activating latent proteinases, and activating growth factors. In a mouse ovalbumin (OVA) asthma model, we increased plasminogen activator activity in the lung by administering exogenous uPA or by using mice genetically deficient in the uPA inhibitor, plasminogen activator inhibitor-1 (PAI-1) to assess the role of this system in asthma pathogenesis. Following intraperitoneal OVA sensitization, mice inhaled OVA plus uPA (500 IU/mouse) or saline by ultrasonic nebulization for 3 weeks. When studied 24 hours after the final exposure, the groups with upregulated plasmin activity had significantly reduced subepithelial fibrosis within the airway walls and had decreased airway hyperresponsiveness to methacholine (AHR). Morphometric analysis showed that subepithelial wall thickening of the bronchi (subepithelial area ratio) was also reduced as was collagen and -smooth muscle actin. Upregulation of plasmin activity also increased the level of hepatocyte growth factor (HGF) activity in BAL fluid, whereas the release of transforming growth factor (TGF)- was decreased. The administration of uPA one week after the last OVA inhalation also significantly reduced lung hydroxyproline content and AHR. These results show that enhancing uPA/plasmin activity lessens the airway remodeling in a murine asthma model.