Cetirizine, an H1 antihistamine, has properties in addition to H1 blockade that may be useful in the treatment of seasonal rhinitis and urticaria. For example, cetirizine has been shown to block the influx of eosinophils into the site of antigen-stimulated skin blisters. Studies with other antihistamines suggest that this is not a universal property of this type of drug. Pretreatment with cetirizine also has been found to block the augmented sensitivity to methacholine that occurs 24 hours after antigen provocation of the nasal mucosa. This reduction takes place despite the absence of an effect on eosinophil influx into this area, and suggests another action of cetirizine. Our study of allergic rhinitis patients examined the effect of cetirizine on early response to nasal challenge with antigen. Cetirizine, although it did not block the release into nasal secretions of histamine, significantly reduced sneezing and decreased levels of albumin and TAME-esterase activity, which are indicators of vascular permeability. Cetirizine also blocked the generation of leukotriene C4. In vitro studies have shown that cetirizine does not block the release of leukotriene from anti-IgE stimulated mast cells, raising the possibility that cells in the nasal mucosa in addition to mast cells generate leukotrienes.