We hypothesized that augmentation of antioxidant defenses with exogenous superoxide dismutase, SOD, an enzyme that provides an initial defense against oxidative injury, would attenuate allergen-induced nasal congestion in the canine model of allergic rhinitis. Nasal congestion was evaluated by the measurements of nasal resistance (R_na) and the volume of nasal passage (V_na). In 5 non-sensitized dogs, 30,000 U of SOD from bovine erythrocytes delivered by aerosol to the nasal passages prior to histamine challenge, reduced the histamine-induced nasal congestion. 30 minute post-challenge, R_na was 1.14±0.2 cmH₂O^.1^-1.min^-1 in the saline-pretreatment study vs. 0.36±0.02 cmH₂O^.1^-1.min^-1 in the SOD-pretreatment study (p<0.05) and V_na was 10.9±0.5 cm³ vs. 17.4±1.3 cm³ (p<0.05), respectively. In 5 sensitized dogs, however, neither an analogous pretreatment with SOD nor intranasal aerosolized pretreatment with 30,000 U of SOD conjugated to polyethylene glycol, PEG-SOD, attenuated ragweed-induced nasal congestion. Also, systemic application of SOD did not attenuate responses to challenges with histamine and ragweed in non-sensitized and sensitized dogs, respectively. The antioxidant-induced attenuation of nasal congestion in non-sensitized dogs confirms validity of the model and indicates the involvement of free radical-mediated damage in the genesis of the histamine-induced congestion. In sensitized dogs, the data do not support the hypothesis that oxidative stress is a clinically significant component of acute ragweed-induced nasal congestion. The data do not support the use of superoxide dismutase for acute protection against allergic rhinitis.