Rationale: The effect of Cetirizine, a selective H1-receptor antagonist, was studied in the Brown Norway rat (BNR) model of trimellitic anhydride (TMA) early and late phase allergic asthma. Methods: Rats were sensitized by 4 hr, weekly applications of 40 mg dry TMA powder on the rats' dorsum for 4 weeks. Two weeks after the final exposure, BNR were challenged with a 10 min, 40 mg/m3 TMA aerosol inhalation exposure and enhanced pause (Penh, an index of airway resistance) was recorded continuously overnight. Cetirizine (15 or 30 mg/kg, i.p.) was given 30 min before or 1 hr after TMA challenge. Results: Both doses of Cetirizine when administered 1 hr after TMA challenge attenuated the late-phase airway response (LAR). Thirty mg/kg Cetirizine administered prior to airway challenge did not inhibit the early-phase airway response (EAR), but again the late-phase airway response was inhibited. Conclusions: The data suggest that the H-1 histamine receptor does not play a major role in the EAR, but is involved in LAR physiological response in the BNR TMA allergic asthma model.