Protease-activated receptors (PARs) and tachykinin-immunoreactive fibers are located in the lung as sentries to respond to a variety of pathological stimuli. The effects of PAR activation on the lung have not been adequately studied. We report on the effects of instilling PAR-activating peptides (PAR-APs, including PAR1-, PAR2-, and PAR4-AP) into the lungs of ventilated or spontaneously breathing mice. PAR2-AP, but not PAR1-AP or PAR4-AP, caused a sharp increase in lung endothelial and epithelial permeability to protein, extravascular lung water, and airway tone. No responses to PAR2-AP were detected in PAR2 knockout mice. In bronchoalveolar lavage, PAR2 activation caused 8-and 5-fold increase in MIP-2 and substance P levels, respectively, and a 12-fold increase in the number of neutrophils. Ablation of sensory neurons (by capsaicin) markedly decreased the PAR2-mediated airway constriction, and virtually abolished PAR2-mediated pulmonary inflammation and edema, as did blockade of NK1 or NK2 receptors. Thus, PAR2 activation in the lung induces airway constriction, lung inflammation, and protein-rich pulmonary edema. These effects were either partly or completely neuropeptide dependent, suggesting that PAR2 can cause lung inflammation by a neurogenic mechanism.