The study will assess the efficacy of OC000459 200 mg twice daily orally in comparison to placebo when subjects are challenged in the Vienna Challenge Chamber for 6 hours. This is a randomised, double blind, placebo controlled, two way crossover evaluation. There will be a screening period of up to three weeks and a washout period of at least one week between the two treatment periods. There will be a follow up one to three weeks after the last dose of study drug.

Inclusion Criteria:

• Males aged 18 to 50 years with a history of symptoms of grass pollen related allergic rhinitis within the previous two years.
• Subjects must be free from significant cardiac, pulmonary, gastrointestinal, hepatic, renal, haematological, neurological and psychiatric disease as determined by history, physical examination and screening investigations.
• FEV1 within normal limits (≥90% of predicted).
• Atopy defined by a positive cutaneous response to mixed grass pollen within the last 12 months or at screening.
• Asymptomatic at screening
• Non smokers for at least the past 12 months

Exclusion Criteria:

• Medical conditions likely to affect the outcome of the study.
• Nasal conditions likely to affect the outcome of the study, i.e. nasal septal perforations, nasal polyps, sinus disease, chronic nasal obstruction, or other nasal diseases.
• Presence of any respiratory disease other than a history of mild stable asthma not requiring treatment and associated with normal lung function
• Immunotherapy treatment course including inhaled or local corticosteroids in the past 28 days.