5-HT (0.01-10μM) contracted isolated hilar bronchus from guinea pig treated with tetrodotoxin in a concentration-dependent manner. 5-HT 2-agonists, such as 5-methoxytryptamine (5-MeOT) and α-methyl-5-HT (α-Me-5-HT), also caused a similar contraction to 5-HT. Other types of 5-HT agonists, such as 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 5-HT1 agonist) of 2-methyl-5-HT (2-Me-5-HT, 5-HT3 agonist) cause no obvious contraction in the preparation. The concentration-response curve of 5-HT was inhibited by the 5-HT2 antagonists, ketanserin, ritanserin, LY53857 and 1-(3-chlorophenyl) piperazine (mCPP) in a concentration-dependent manner, while the maximum response of 5-HT was reduced by them. These results suggest that 5-HT-induced contraction may be mediated through an action on 5-HT2-like receptors, and that the receptors mediating the responses of 5-HT in hilar bronchus are different from those in other tissues in their pharmacological profile.