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

Inclusion Criteria

• Males and females aged 12-76 years with documented clinical history of symptomatic seasonal allergic rhinitis associated with mountain cedar pollen for the last 2 years.
• Evening reflective Total Nasal Symptom Score (p.m.rTNSS) at least 42 out of a maximum potential score of 84 over the 7-day run-in period, or > or = 6 out of a maximum score of 12 on each of the last 4 consecutive days during the run-in period.

Exclusion Criteria

• Non-allergic rhinitis.
• Severe physical nasal obstruction.
• Acute or significant chronic sinusitis.
• Bacterial or viral infection of the upper or lower respiratory tract, nasal sinuses, or middle ear.
• Ongoing chronic respiratory disorders.
• Asthma requiring use of short-acting beta2-agonists > 2 times a week or any asthma treatment other than inhaled short-acting beta2-agonists.
• Diagnosis of aspirin or non-steroidal anti-inflammatory drug (NSAID)-induced asthma.
• Nasal biopsy or surgery, sinus surgery, or perforation within the 8 weeks prior to the Screening Visit.
• Ocular surgery within the 8 weeks prior to the Screening Visit.
• Ocular infections (bacterial or viral) within the 4 weeks before screening.
• Use of forbidden medications (prescribed or over-the-counter [OTC])
• Congenital or acquired severe immunodeficiency or known Human immunodeficiency virus (HIV)infection.