5-hydroxytryptamine (5-HT) has been reported to show some effects in respiratory tissues by activation of different subtype receptors. It has been demonstrated that 5-HT2 receptor activation causes in vivo and in vitro airways contraction and enhances effects of cholinergic nerve-mediated responses, whereas 5-HT1 receptor activation seems to be related to a relaxant effect. Moreover, in isolated guinea pig ascendens colon preparations 5-HT1 activation causes relaxation by involvement of nitric oxide (NO). The aim of this study was to investigate the effects of 5-HT1 receptor activation in guinea pig trachea as well as NO probable role in this activation. In tissues pretreated with both ketanserin (10 microM), an antagonist of 5-HT2 receptors, and ondansetron (10 microM), an antagonist of 5-HT3 receptors, 5-HT (from 10 nM to 10 mM) relaxed guinea pig trachea precontracted with acetylcholine (ACh, 100 microM). Carboxamidotryptamine (5-CT, from 10 nM to 10 mM), an agonist of 5-HT1 receptors, as well relaxed guinea pig trachea precontracted with ACh (100 microM). A pretreatment with NAN-190 (from 10 nM to 100 microM), a 5-HT1A selective antagonist, reduced the 5-HT and 5-CT relaxant effects but only at very high concentrations. Finally, a pretreatment with L-nitro-arginine-methyl-ester (L-NAME, 1 mM), an inhibitor of NO-synthase, and L-arginine (L-ARG, 1 mM), a precursor of NO synthesis, did not modify 5-HT and 5-CT responses in guinea pig trachea. In conclusion, this study suggests a 5-HT relaxant activity in guinea pig trachea via a 5-HT1 receptor activation without any NO pathway involvement. However, further investigations are needed to clarify which 5-HT1 receptor subtype is involved in 5-HT relaxant effect.