 This is a single-center, prospective, randomized, double-blind, placebo-controlled, two-way crossover study. A live cat challenge model will be used for exposing subjects to Fel d1. Subjects who test positive for cat allergy symptoms during a priming cat exposure challenge will be eligible to enter the treatment phase of the study. Baseline efficacy measures will be obtained prior to the dispensing of study medication during both treatment periods. Cat challenges will be initiated 1.5 hours following treatment with study medication. 