1 Relaxant and contractile effects of the tethered ligand domain sequences of murine PAR-1, PAR-2, PAR-3 and PAR-4, and of the proteases thrombin and trypsin were examined in mouse isolated tracheal preparations. The epithelium- and cyclo-oxygenase-dependence of these effects and the potential modulatory effects of respiratory tract viral infection were also investigated.2 In carbachol-contracted preparations, trypsin, thrombin, and the tethered ligand domain sequences of murine PAR-1 (SFFLRN-NH2), PAR-2 (SLIGRL-NH2) and PAR-4 (GYPGKF-NH2), but not PAR-3 (SFNGGP-NH2), induced transient, relaxant responses that were abolished by the cyclo-oxygenase inhibitor indomethacin.3 Repeated administration of SFFLRN-NH2, SLIGRL-NH2 or GYPGKF-NH2 (30 mu M) was associated with markedly diminished relaxation responses (homologous desensitization), although there was no evidence of cross-desensitization between these peptides.4 The tethered ligand domain sequences for PAR-1 and PAR-4 induced a rapid, transient contractile response that preceded the relaxant response. Contractions were not inhibited by indomethacin and were not induced by either thrombin or trypsin.5 Influenza A virus infection did not significantly affect the responses induced by either the proteases or peptides. Furthermore, epithelial disruption caused by mechanical rubbing had no significant effect on responses to these PAR activators in preparations from either virus- or sham-infected mice.6 In summary, the proteases trypsin and thrombin, and peptide activators of PAR-1, PAR-2 and PAR-4 induced relaxant responses of mouse isolated tracheal smooth muscle preparations, which were mediated by a prostanoid, probably PGE(2). Interestingly, PAR-mediated relaxations were not significantly diminished following acute damage to the epithelium caused by mechanical rubbing and/or the respiratory tract viral pathogen, influenza A.