The ragweed- and histamine-induced decreases in nasal patency in cohorts of ragweed-sensitized and non-sensitized dogs were assessed. The volume of nasal airways, V_NA, was assessed by acoustic rhinometry and resistance to airflow, R_NA, by anterior rhinomanometry. Histamine delivered to the nasal passages of 5 dogs caused a rapid and prolonged increase in R_NA (0.75 ± 0.26 to 3.56 ± 0.50 cm H₂O/L/min); an effect that was reversed by intranasal delivery of aerosolized phenylephrine. Ragweed challenge in 5 ragweed-sensitized dogs increased R_NA from 0.16 ± 0.02 to 0.53 ± 0.07 cm H₂O/L/min and decreased Vna from 12.5 ± 1.9 to 3.9 ± 0.3 cm³ whereas administration of saline aerosol neither increased R_NA nor decreased V_NA. Prior administration of d-pseudoephedrine (30 mg, p.o.) attenuated the ragweed-induced increase in R_NA and decrease in V_NA. Ragweed challenge changed neither R_NA nor V_NA in 4 non-sensitized dogs. Mediator-induced nasal congestion and allergen-induced allergic rhinitis in ragweed-sensitized dogs, which exhibit symptoms similar to human disease, can be used in the evaluation of safety and efficacy of anti-allergic activity of potential drugs.