We investigated the function of proteinase-activated receptor-1 (PAR-1) in the regulation of pulmonary microvascular permeability in response to thrombin challenge using PAR-1 knockout mice (−/−). Lungs were isolated and perfused with albumin (5 g/100 ml)-Krebs solution at constant flow (2 ml/min). Lung wet weight and pulmonary artery pressure (P_pa) were continuously monitored. We determined the capillary filtration coefficient (K_fc) and ¹²⁵I-labeled albumin (BSA) permeability-surface area product (PS) to assess changes in pulmonary microvessel permeability to liquid and albumin, respectively. Normal and PAR-1-null lung preparations received in the perfusate: 1) thrombin or 2) selective PAR-1 agonist peptide (TFLLRNPNDK-NH₂). In control PAR-1 (+/+) mouse lungs, ¹²⁵I-albumin PS and K_fc were significantly increased over baseline (by ∼7- and 1.5-fold, respectively) within 20 min of α-thrombin (100 nM) challenge. PAR-1 agonist peptide (5 μM) gave similar results, whereas control peptide (5 μM; FTLLRNPNDK-NH₂) was ineffective. At relatively high concentrations, thrombin (500 nM) or PAR-1 agonist peptide (10 μM) also induced increases in P_pa and lung wet weight. All effects of thrombin (100 or 500 nM) or PAR-1 agonist peptide (5 or 10 μM) were prevented in PAR-1-null lung preparations. Baseline measures of microvessel permeability and P_pa in the PAR-1-null preparations were indistinguishable from those in normal lungs. Moreover, PAR-1-null preparations gave normal vasoconstrictor response to thromboxane analog, U-46619 (100 nM). The results indicate that the PAR-1 receptor is critical in mediating the permeability-increasing and vasoconstrictor effects of thrombin in pulmonary microvessels.