The histamine H3-agonist (R)-alpha-methylhistamine (alpha-MeHA) caused a dose-dependent inhibition of vagally-mediated contraction of a guinea-pig tracheal tube preparation but did not alter tracheal contraction induced by exogenously-applied acetylcholine. Blockade of H1- and H2-histamine receptors, and alpha- and beta-adrenoceptors failed to prevent the inhibitory effect of alpha-MeHA, whereas the specific H3-antagonist thioperamide prevented the effect of alpha-MeHA on tracheal contraction. In the presence of H1- and H2-receptor antagonists, histamine also inhibited vagally-mediated tracheal contraction. The inhibitory effect of alpha-MeHA was greater with preganglionic (vagus nerve) stimulation than with postganglionic stimulation by electrical field stimulation, suggesting that H3-receptors are localized both to cholinergic ganglia and to post-ganglionic nerve-endings. Our results suggest that H3-receptors exist on the vagus nerve which modulate cholinergic neurotransmission in the airways.