Cetirizine is a potent, selective H1 histamine receptor antagonist. The effect of oral and inhaled eetirizine was assessed on the early bronchoconstrictor response to inhaled allergen in 10 mild atopie asthmatic patients in a double-blind, randomized, plaeebo controlled trial. All were sensitive to Dermatophagoides pleronyssinus and this was used as the provoking allergen. The geometric mean PD20 FEV] values obtained at allergen challenge were measured as cumulative breath units (c.b.u.) and following oral cetirizine, inhaled cetirizine and placebo were 124–5, 75–7 and 76–7 c.b.u. respectively. These did not differ significantly. We conclude that neither oral nor inhaled cetirizine significantly attenuates the early response to inhaled allergen in atopie asthmatic subjects. However, the method of repeated allergen challenge is likely to be relatively insensitive. Clinical and Experimental Allergy, [o]. 23, pp. 528–531.