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Background: Nasal congestion in allergic rhinitis (AR) is caused by vascular hyperpermeability and vascular relaxation of the nasal mucosa. We previously detected high levels of a lipoxygenation metabolite of dihomogammalinolenic acid, 15-hydroxy-8Z,11Z,13E-eicosatrienoic acid (15-HETrE) in the nasal lavage fluid of AR model mice. Here, we investigated the effects of 15-HETrE on vascular functions associated with nasal congestion. Methods: We measured 15-HETrE levels in the nasal lavage fluid of ovalbumin-induced AR model mice and nasal discharge of patients with AR. We also assessed nasal congestion and vascular relaxation in mice. Vascular contractility was investigated using isolated mouse aortas. Results: Five ovalbumin challenges increased 15-HETrE levels in AR model mice. 15-HETrE was also detected in patients who exhibiting AR-related symptoms. Intranasal administration of 15-HETrE elicited dyspnea-related behavior and decreased the nasal cavity volume in mice. Miles assay and whole-mount immunostaining revealed that 15-HETrE administration caused vascular hyperpermeability and relaxation of the nasal mucosa. Intravital imaging demonstrated that 15-HETrE relaxed the ear vessels that were precontracted via thromboxane receptor stimulation. Moreover, 15-HETrE dilated the isolated mouse aortas, and this effect was attenuated by K+ channel inhibitors and prostaglandin D2 (DP) and prostacyclin (IP) receptor antagonists. Additionally, vasodilatory effects of 15-HETrE were accompanied by an increase in intracellular cAMP levels. Conclusions: Our results indicate that 15-HETrE, whose levels are elevated in the nasal cavity upon AR, can be a novel lipid mediator that exacerbates nasal congestion. Moreover, it can stimulate DP and IP receptors and downstream K+ channels to dilate the nasal mucosal vasculature.