Numerous studies have compared the duration of the cutaneous effect of cetirizine and loratadine. We assessed their nasal effects 24 hours after administration in patients with allergic rhinitis, using a randomized, double-blind, crossover, placebo-controlled trial. Nasal challenge was performed by nebulization of increasing doubling dosages of histamine (0.04-1.28 mg/nostril) in 12 patients (seven males, five females, aged 31 +/- 7 years). Nasal airway resistance was measured by posterior rhinomanometry 24 hours after intake of cetirizine (10 mg), loratadine (10 mg), or placebo. Baseline nasal airway resistance was identical on all study days (2.86 +/- 0.10 cm H2 O/L per second). Twenty-four hours after intake, the dose-response curve of nasal obstruction to histamine was significantly lower after treatment with cetirizine compared with placebo (P < 0.05). However, although the curve was lower on loratadine than on placebo, the curves did not differ significantly. In conclusion, our study shows significant efficacy of cetirizine, but not of loratadine, in the nose at 24 hours after a single dose. This suggests that the nasal action of cetirizine is longer lasting than that of loratadine in patients with allergic rhinitis.