||In allergic rhinitis, a nasal H1-antihistamine spray seems to be well suited for usage on an as-needed basis, because it has a quick onset of action, and many patients prefer to take medicine only when they have symptoms. It is a prerequisite, however, that nasal hypersecretion during a rhinitis episode does not significantly reduce the efficacy of intranasal treatment by washing away the drug before it reaches the H1-histamine receptors. In order to investigate this problem, we have induced nasal hypersecretion with a methacholine challenge in one experiment and in four experiments we have washed the nasal cavities 0.5 min. before, 5 min. before, 0.5 min. after and 5 min. after intranasal use of the H1-antagonist, levocabastine. The symptom response to a subsequent histamine challenge was used as the effect parameter. Levocabastine reduced the number of histamine-induced sneezes with 81% (p < 0.0001) and the secretion weight with 62% (p < 0.001) compared with placebo. Neither methacholine-induced hypersecretion nor washing the nose with saline reduced the efficacy of the antihistamine spray. We conclude that experimentally induced nasal hypersecretion does not reduce the efficacy of the antihistamine spray, and probably the same applies to rhinorrhea during an acute episode of allergic rhinitis.