Controlled conditions of the EEU allow a reliable assessment of the efficacy and a determination of the action onset, action intensity and duration of effect of levocetirizine and cetirizine in order to establish the relative efficacy of these two drugs available for the treatment of SAR.

Inclusion Criteria:

• Male/Female, aged >= 16 years
• seasonal allergic rhinitis that required pharmacologic therapy each year during the last 2 ragweed pollen seasons
• documented seasonal allergy to ragweed pollen
• total symptom score of at least 18 points.

Exclusion Criteria:

• nasal anatomic deformities ? 50% obstruction
• acute sinusitis within 30 days of Period 2
• initiated or advanced an immunotherapy regimen
• immunotherapy injections within 48 hours of pollen exposure
• impaired hepatic function
• history of malignancy
• intolerance to histamines
• asthma requiring medication more than occasionally