Rapid relief of symptoms should be one of the primary goals of treatment for allergic rhinitis (AR). The onset and duration of action of olopatadine hydrochloride nasal spray, 665 mcg (OLO; Patanese), for seasonal AR (SAR) was evaluated in this study. This study was performed to determine the onset and duration of action of OLO compared with placebo spray, with mometasone furoate monohydrate, 50 mcg (MM; Nasonex), as a reference standard. This was a single center, single-dose, randomized, double-blinded parallel-group environmental exposure chamber study. Patients were primed at two 2-hour priming visits. Eligible patients were randomized to OLO, placebo spray, or MM, 2 sprays/nostril. Allergy symptoms (sneezing, runny, itchy, and stuffy nose) were rated by patients at 16 time points during 12 hours after dosing and patient satisfaction was assessed at 4 and 12 hours postdose. Safety was assessed by a review of adverse events, cardiovascular and nasal examination parameters. Four hundred twenty-five adult patients were randomized. OLO was superior to placebo spray in reducing total nasal symptoms (TNSS) within 30 minutes after dosing and maintained superiority for at least 12 hours (p < 0.05). The onset of MM was not observed until 150 minutes postdose and was smaller in magnitude compared with OLO. OLO was superior to both placebo spray (p < 0.0001) and MM (p < 0.05) in patient satisfaction. Treatment was well-tolerated with no safety concerns. OLO is superior to placebo spray and MM in reducing allergy symptoms; OLO has a rapid onset of action and a duration of effect of at least 12 hours.