This was a study to measure the ability of desloratadine and levocetirizine to decrease nasal obstruction in subjects who had study-induced allergic rhinitis. Study participants had allergic rhinitis symptoms induced in a Vienna Challenge Chamber and then received desloratadine or levocetirizine for 8 days. After at least a 10 to 35-day washout period (time when no drug is given), subjects received the opposite treatment for 8 days. Subjects had their total nasal symptom measured.on the 8th day of each treatment period.

Inclusion Criteria:

• 18 to 45 years of age
• Were free of any clinically significant disease that would interfere with study evaluations.
• Demonstrated hypersensitivity to the grass allergen used in the chamber unless confirmed previously within 12 months.
• Hypersensitivity to the allergen was documented by a positive response to the skin prick test with a wheal diameter at least 3 mm larger than the diluent control and by a radioallergosorbent test (RAST) class of >=2 (rating scale 0 -6).
• Had a history of SAR for at least 2 years, as diagnosed by the investigator, another physician, or subject-provided history.
• Reported having shown therapeutic efficacy with previous use of an antihistamine (without a decongestant).
• At the screening allergen provocation, the subject had a nasal obstruction symptom severity score of at least moderate (>=2) and a decrease from baseline in nasal airflow (mL/sec) measured by rhinomanometry of >=30% within 2 hours of allergen exposure. Also, the subject had a nasal obstruction symptom severity score of none or mild (<=1) prior to exposure to allergen.

Exclusion Criteria:

• Women who were pregnant or nursing.
• Had a body mass index (BMI) >=30 kg/m^2.
• Had asthma; were being treated with inhaled or oral corticosteroids, chromones, theophylline, leukotriene inhibitors, or short-acting inhaled β2-agonists (except during or immediately after the allergen exposures); and could not go through the washout periods and the entire study without needing these medications.
• Developed wheezing or dyspnea during the screening allergen exposure.
• Had a respiratory infection during the 4 weeks prior to pre-dose evaluations.
• Had any clinically significant deviation from normal in the physical examination that, in the investigator's judgment, could interfere with the study evaluation or affect subject safety.
• Had any history of or laboratory evidence of hepatic failure or renal failure with a glomerular filtration rate (GFR) known to be <30 mL/min.
• Had a known potential for hypersensitivity, allergic, or idiosyncratic reaction to the study drug or excipients.
• Had nasal structural abnormalities, including large nasal polyps and marked septum deviation, that significantly interfered with nasal airflow.