ABSTRACT-We examined whether nasal hyperresponsiveness to leukotriene (LT) D4 is seen in our allergic rhinitis model, which showed sneezing and biphasic nasal blockage by repeated antigen inhalation challenge, and whether a dilatation of mucosal blood vessels contributes to this hyperresponsiveness. Nasal blockage [increase of specific airway resistance (sRaw)] was indexed as nasal (hyper)responsiveness. The sensitized -challenged guinea pig showed a remarkable dose-dependent increase in sRaw by intranasal instillation of LTD 4 (10 m l /nostril) at 10 -10 to 10 -6 M 10 h and 2 days but not 7 days after the challenge. The increase in sRaw induced by LTD4 was largely blocked by pranlukast or naphazoline, and this was dosedependently suppressed by N M -nitro-L-arginine methyl ester. Sodium nitroprusside induced an elevation of sRaw in the sensitized -challenged animal in the hyperresponsiveness state, but the degree did not differ from that in the non-sensitized -non-challenged group. The amount of NO2-and NO3 -in nasal cavity lavage fluid after LTD4 instillation in the sensitized -challenged animal in the hyperresponsiveness state was significantly greater than that before the instillation. These results demonstrate that the hyperresponsiveness to LTD4 acquired by repeated antigen challenge is mainly due to dilatation of nasal blood vessels, which can be related to hyperproduction of nitric oxide through cysteinyl LT1-receptor activation.Keywords: Hyperresponsiveness, Allergic rhinitis, Leukotriene D4, Nasal blockage, Nitric oxide Cysteinyl leukotrienes (CysLTs: LTC4, LTD4 and LTE4) are a family of potent inflammatory mediators that appear to contribute to the pathophysiologic features of allergic rhinitis. It has been reported that CysLTs were detected in the nasal washings following antigen challenge of subjects with allergic rhinitis (1, 2) and that treatment with a 5-lipoxygenase inhibitor and a CysLT1-receptor antagonist modified allergen-induced nasal mucosa swelling via dilatation of nasal capacitance blood vessel and increases in nasal capillary permeability (3 -5), suggesting that CysLTs act as significant inflammatory mediators in allergic rhinitis and that allergen-induced vascular changes are predominantly induced by CysLTs.To date, clinical examinations revealed that allergic rhinitis patients show obviously increased nasal blockage in response to topical LTD4, a phenomenon known as nasal hyperresponsiveness (6 -8). On the other hand, although very high doses of topical LTD 4 have been reported to increase either nasal blockage or nasal vascular permeability in non-sensitized animals (9, 10), it has not been determined whether a higher response to the agonist occurs in the allergic rhinitis model. Due to the limitation of clinical research and insufficient information obtained from normal animals, the mechanism of nasal hyperresponsiveness to LTD4 in allergic rhinitis patients remains unclear. The use of an experimental allergic rhinitis model that closely resembles the human case indispensably resolv...