Protective roles for protease-activated receptor-2 (PAR 2 ) in the airways including activation of epithelial chloride (Cl Ϫ ) secretion are based on the use of presumably PAR 2 -selective peptide agonists. To determine whether PAR 2 peptide-activated Cl Ϫ secretion from mouse tracheal epithelium is dependent on PAR 2 , changes in ion conductance across the epithelium [short-circuit current (I SC )] to PAR 2 peptides were measured in Ussing chambers under voltage clamp. In addition, epitheliumand endothelium-dependent relaxations to these peptides were measured in two established PAR 2 bioassays, isolated ring segments of mouse trachea and rat thoracic aorta, respectively. Apical application of the PAR 2 peptide SLIGRL caused increases in I SC , which were inhibited by three structurally different neurokinin receptor-1 (NK 1 R) antagonists and inhibitors of Cl Ϫ channels but not by capsaicin, the calcitonin generelated peptide (CGRP) receptor antagonist CGRP 8 -37 , or the nonselective cyclooxygenase inhibitor indomethacin. Only high concentrations of trypsin caused an increase in I SC but did not affect the responses to SLIGRL. Relaxations to SLIGRL in the trachea and aorta were unaffected by the NK 1 R antagonist nolpitantium (SR 140333) but were abolished by trypsin desensitization. The rank order of potency for a range of peptides in the trachea I SC assay was 2-furoyl-LIGRL Ͼ SLCGRL Ͼ SLI-GRL ϭ SLIGRT Ͼ LSIGRL compared with 2-furoyl-LIGRL Ͼ SLIGRL Ͼ SLIGRT Ͼ SLCGRL (LSIGRL inactive) in the aorta relaxation assay. In the mouse trachea, PAR 2 peptides activate both epithelial NK 1 R coupled to Cl Ϫ secretion and PAR 2 coupled to prostaglandin E 2 -mediated smooth muscle relaxation.