OBJECTIVE: Seasonal allergic rhinitis (SAR) is a highly prevalent disease. This study was conducted to evaluate the onset and duration of action of three concentrations of olopatadine nasal spray. METHODS: This was a randomized, double-blind, single-dose, placebo-controlled study, conducted in an environmental exposure chamber in patients with SAR. A total of 320 patients were exposed to ragweed allergen in the chamber and randomized to olopatadine nasal spray 0.2%, 0.4%, 0.6%, or placebo nasal spray. Symptoms (sneezing, runny, itchy, and stuffy nose) were self-assessed during a 12-hour study period. RESULTS: All concentrations of olopatadine nasal spray provided clinically meaningful reductions in total nasal symptom scores at 30 minutes compared to the placebo. Olopatadine nasal spray 0.6% was significantly more effective (P < 0.05) than placebo nasal spray at all time-points starting at 90 minutes post-dose and continuing over 12 hours. CONCLUSIONS: Olopatadine nasal spray 0.6% demonstrated a fast onset of action and maintained an effect for at least 12 hours after dosing.