Data from in vitro, in vivo, and ex vivo studies suggest that second-generation antihistamines have a number of antiallergic, anti-inflammatory properties that appear to be independent of their H1-blockade activity. First-generation antihistamines also have antiallergic, anti-inflammatory properties, as suggested by the studies with azatadine, chlorpheniramine, mepyramine, and promethazine; most other first-generation antihistamines have not been studied for these properties. In vitro studies have shown that H1-antihistamines reduce the release of proinflammatory mediators from mast cells and basophils, the chemotaxis and activation of inflammatory cells (especially eosinophils), and the expression of adhesion molecules induced by immunological and nonimmunological stimuli in epithelial cell lines. Nasal allergen challenge models have similarly demonstrated that H1-antihistamines inhibit mediator release from mast cells and basophils, and that they decrease inflammatory cell infiltration and the expression of adhesion molecules on epithelial cells. The results of published studies of the effects of H1-antihistamines on nasal allergic inflammation in humans have been summarized in this chapter. Recent investigations indicate that H1-antihistamines may modulate airway inflammation by downregulating the activity of airway epithelial cells, which have an important role in allergic airway inflammation. The modulation of adhesion molecules and of inflammatory cell infiltration by H1-antihistamines may be beneficial during long-term treatment in patients with allergic rhinitis. The rationale for this hypothesis is the persistence of inflammation on the nasal epithelial cells even when patients are symptom-free (16). All of the events affected by H1-antihistamines are important in the allergic inflammation cascade. The underlying mechanisms for such effects remain unclear, but are unrelated to H1-antagonist activity. Several studies have demonstrated that H1-antihistamines can form an ionic association with cell membranes and inhibit calcium ion influx into the mast cell or basophil plasma membrane, or inhibit Ca2+ release within the cells, and may therefore influence the signal transduction pathways. However, these effects appear to occur at concentrations higher than those achieved in therapeutic practice (126-128). It has recently been hypothesized that the anti-inflammatory activity of H1-antihistamines may be a consequence of their ability to influence the activation of genes responsible for the expression and synthesis of proinflammatory mediators (129). The contribution of the antiallergic effects of H1-receptor antagonists to their clinical efficacy is not fully understood. There have been no data suggesting that H1-antihistamines with well-documented antiallergic properties are superior to the others for which such properties have not been as extensively investigated. Additional studies are needed to elucidate the mechanisms(s) by which H1-antihistamines exert anti-inflammatory effects. This knowledge might lead to the development of novel therapies with more potent and specific anti-inflammatory effects.