The effect of intranasal administration of levocabastine, a new selective H1-receptor antagonist, was investigated in a nasal provocation test (NPT) performed with allergens. The NPT allowed a quantitative estimation of the nasal allergic threshold (concentration of allergen necessary to trigger the reaction). In addition, the intensity of the three major rhinitis symptoms (obstruction, rhinorrhea and sneezing) was determined. Twelve adult patients, allergic to grass pollen, underwent a first NPT without pretreatment ('initial NPT'); the NPT was then repeated after the single intranasal administration of either placebo, 8 mg disodium cromoglycate (DSCG) or 0.2 mg levocabastine in a double-blind random order. The NPTs gave reproducible results since both the threshold and symptom intensities were similar in the initial NPT and in the NPT performed after placebo. The reaction threshold increased in 8/12 patients after DSCG (0.05 less than P less than 0.1) and in 9/12 patients after levocabastine (P less than 0.05). Levocabastine clearly inhibited rhinorrhea (P less than 0.001) and sneezing (P less than 0.02) but did not influence the nasal obstruction. DSCG inhibited rhinorrhea only (P less than 0.01). The intranasal administration of levocabastine might be useful in the treatment of allergic rhinitis.