This study will assess the efficacy and safety of different doses of ACT-129968 in adult patients with seasonal allergic rhinitis, due to mountain cedar pollen.

Inclusion Criteria:

• Males and females aged 18 to 70 years.
• Clinical history of symptomatic seasonal allergic rhinitis associated with mountain cedar pollen for the last two years.
• Adequate exposure to mountain cedar pollen.
• Positive skin prick test to mountain cedar allergen within 12 months of screening.
• Sufficient nasal symptom score during a run-in period.

Exclusion Criteria:

• Non-allergic rhinitis.
• Bacterial or viral respiratory tract infection.
• Chronic respiratory disorders.
• Asthma requiring treatment other than inhaled short-acting Beta2-agonists.
• Ocular infections or surgery, nasal obstruction, nasal biopsy or surgery, sinus surgery or perforation.
• Smoking within the last year.
• Ongoing or recent treatment for seasonal allergic rhinitis.
• Initiation of allergen immunotherapy within 6 months of screening.