Effect of eetirizine. a potent and specific H1 receptor antagonist, was examined on platelet activating factor-induced bronchoconstrietion in 10 patients (5 male, mean [s.e.m.] aged 37 4 [3–6] years) with mild asthma in a placebo controlled, double-blind cross-over study. Airway responses were assessed by measuring specific airway conductance (SGaw). Patients were challenged with a single dose (12–96 μG) of PAF that had previously produced a 35% fall in SGaw. PAF challenges were performed after single dose (15 mg) and I week's treatment (15mg twice daily) of cetirizine. There was no significant difference in pre-and post-treatment baseline values of SGaw on different study days and the percentage changes after cetirizine were 38 7 (701) and 45–6 (5–52) eompared to 50 2 (2–89) and 43–9 (7–26) with placebo respectively. Similarly mean (s.e.m.) area under eurve (AUC-SGaw/time course response) was 391 (143) and 514 (85) with cetirizine compared to 565 (37) and 461 (94) with placebo respectively. The difference was not statistically significant. There was no difference in facial flushing and feeling of warmth between cetirizine and placebo. We conclude that PAF induced bronehoconstriction in humans is not mediated by histamine release and that H1 receptor antagonists do not modify PAF indueed bronehoeonstriction.