1. The aim of the present study was to investigate the effects of bradykinin and [des-Arg9]-bradykinin and their relaxant mechanisms in the mouse isolated trachea. 2. In the resting tracheal preparations with intact epithelium, bradykinin and [des-Arg9]-bradykinin (each drug, 0.01-10 microM) induced neither contraction nor relaxation. In contrast, bradykinin (0.01-10 microM) induced concentration-dependent relaxation when the tracheal preparations were precontracted with methacholine (1 microM). The relaxation induced by bradykinin was inhibited by the B2 receptor antagonist, D-Arg0-[Hyp3,Thi5,D-Tic7,Oic8]-bradykinin (Hoe 140, 0.01-1 microM) in a concentration-dependent manner whereas the B1 receptor antagonist, [des-Arg9,Leu8]-bradykinin (0.01-1 microM), had no inhibitory effect on bradykinin-induced relaxation. [des-Arg9]-bradykinin (0.01-10 microM) also caused concentration-dependent relaxation after precontraction with methacholine. The relaxation induced by [des-Arg9-bradykinin was concentration-dependently inhibited by the B1 receptor antagonist, [des-Arg9,Leu8]-bradykinin (0.01-1 microM), whereas the B2 receptor antagonist, Hoe 140 (0.01-1 microM) was without effect. 3. In the presence of the cyclo-oxygenase inhibitor, indomethacin (0.01-1 microM), the relaxations induced by bradykinin and [des-Arg9]-bradykinin were inhibited concentration-dependently. 4. Two nitric oxide (NO) biosynthesis inhibitors NG-nitro-L-arginine methyl ester (L-NAME, 100 microM) and NG-nitro-L-arginine (L-NOARG, 100 microM) had no inhibitory effects on the relaxations induced by bradykinin and [des-Arg9]-bradykinin. Neither did the selective inhibitor of the soluble guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10 microM) inhibit the relaxations induced by bradykinin and [des-Arg9]-bradykinin. 5. Prostaglandin E2 (PGE2, 0.01-33 microM) caused concentration-dependent relaxation of the tracheal preparations precontracted with methacholine. Indomethacin (1 microM) and ODQ (10 microM) exerted no inhibitory effects on the relaxation induced by PGE2. 6. The NO-donor, sodium nitroprusside (SNP; 0.01-100 microM) also caused concentration-dependent relaxation of the tracheal preparations precontracted with methacholine. ODQ (0.1-1 microM) concentration-dependently inhibited the relaxation induced by SNP. 7. These data demonstrate that bradykinin and [des-Arg9]-bradykinin relax the mouse trachea precontracted with methacholine by the activation of bradykinin B2-receptors and B1-receptors, respectively. The stimulation of bradykinin receptors induces activation of the cyclo-oxygenase pathway, leading to the production of relaxing prostaglandins. The NO pathway is not involved in the bradykinin-induced relaxation. The relaxation caused by NO-donors in the mouse trachea is likely to be mediated via activation of soluble guanylate cyclase.