The antihistaminic effects of aqueous and macerated extracts, essential oil, 20 nM chlorpheniramine, and saline were tested by performing the cumulative log concentration-response curves of histamine-induced contraction of isolated guinea pig tracheal chains incubated with three different conditions including: (1) 1.4 microM indomethacin, (2) indomethacin, 1 microM propranolol, and 10 nM atropine, and (3) indomethacin and propranolol (for each group n = 8). The results showed clear parallel rightward shifts in histamine-response curves obtained in the presence of macerated extract in group 2, aqueous extract in group 3, and essential oil in groups 2 and 3 experiments compared with the curves obtained in the presence of saline. The EC50 (effective concentration of histamine causing 50% of maximum response) obtained in the presence of essential oil, extracts, and chlorpheniramine in all three sets of experiments were significantly higher than that of saline (P<0.05 to p<0.001). The maximum response obtained in the presence of aqueous extract in group 3 compared to group I and that of macerated extract in group 2 compared to the other two sets of experiments were improved. These results indicated a competitive antagonistic effect of Bunium persicum at histamine H1 receptors.