Histamine, 2-methylhistamine (a specific H1-agonist), 5-HT, PGF2alpha, SRS-A, bradykinin (BK) and carbachol contract bronchial and tracheal smooth muscles of the horse. Isoprenaline, PGE1, E2, dimaprit and 4-methylhistamine (last two = specific H2-agonists) relaxed airways which were partially contracted to carbachol. Mepyramine (a specific H1-antagonist) selectively antagonized contractions to histamine. In the presence of mepyramine, histamine caused relaxation of airways partially contracted to carbachol. Metiamide and burimamide (specific H2-antagonists) specifically antagonized or reversed histamine-induced bronchorelaxation. However the H2-antagonists, indomethacin and propranolol each failed to block histamine-induced relaxations in trachea. Thus, the results of this study show: (i) preponderence of H1-receptors-mediating contractions in horse airways; (ii) presence of H2-receptors-mediating bronchorelaxation and (iii) the existence of an atypical (relaxant) response (resistance to H2-antagonists; indomethacin and propranolol) in the horse trachea.