This study addressed the question whether proximal and distal guinea pig tracheal segments respond differently to contractile agents. Using a perfused trachea set-up, histamine, KCl or the cyclo-oxygenase inhibitor, indomethacin, could be administered selectively to the mucosa (at the inside) or the serosa (at the outside) of the tracheal segments. Proximal parts contracted significantly more (40-60%) than distal parts when 1 mM histamine was administered to the mucosal or serosal side or when KCl (50 mM) was added to the serosal side. When histamine was administered to the mucosal side of epithelium-denuded segments, the contractions were twice as high in proximal than in distal parts (3057 vs. 1526 mg). Inhibition of tracheal cyclo-oxygenase with indomethacin at the mucosal side increased proximal and distal reactivity to mucosally administered histamine to the same extent. Serosal administration of indomethacin, however, increased histamine reactivity only in proximal segments (from 2690 to 5180 mg). In the latter segments, subsequent administration of histamine to the serosal side further increased the contraction, while serosal histamine in the absence of serosal indomethacin produced a relaxation (net difference of 4672 mg). In conclusion, the higher intrinsic contractility of proximal tracheal segments is counteracted by serosal cyclo-oxygenase products.