Allergic rhinitis is a common condition characterize by inflammation of the upper airways. Third generation antihistamines have been demonstrated effective in the treatment of this condition. Allergen challenge can be use to assess the effects of drugs in allergic rhinitis, adenosine monophosphate may also be used as a means to investigate these effects but as yet its effects have yet to be compared to allergen challenge. We intend to compare the effects of levocetirizine at a single dose of 5mg on allergen and AMP challenge compared to placebo in a double blind cross-over study. The study will include 20 patients with allergic rhinitis. Each patient will have allergen and AMP challenge on placebo and active treatment. The primary outcome variable will be the change in concentration of AMP/Allergen required to produce a 20% drop in nasal flow as manifest by peak nasal inspiratory flow. A 1 doubling dose change in concentration of challenge medium to cause a 20% drop in nasal flow will be deemed significant. We will also measure time to recovery after both challenges. AMP challenge is a safe alternative to allergen challenge and does not have the risk of anaphylaxis.