[en] To investigate the possible role of mast cell or basophil histamine release in mediating platelet-activating factor (PAF) airway obstruction, we studied the effect of inhaled PAF (30 micrograms, single dose) on plasma histamine, bronchial caliber, and leukocyte and platelet counts in six patients with mild or moderate allergic asthma (mean age, 27 +/- 1.3 years; mean FEV1, 95 +/- 5 percent of predicted; mean PC20 methacholine, 1.46 +/- 0.36 mg/ml). Specific conductance (SGaw) FEV1, FEF25-75 percent, differential leukocyte and platelet counts, and plasma histamine (radioimmunoassay) were measured before and 5, 10, 15, and 20 min after PAF inhalation. Mean basal plasma histamine level was 0.28 +/- 0.04 ng/ml. Inhalation of PAF caused a fall in SGaw peaking at 5 min (43 +/- 9 percent) and a fall in FEV1 and FEF25-75 peaking at 10 min (19 +/- 10 percent and 30 +/- 13 percent, respectively). There was also a rapid and transient fall in circulating neutrophils at 5 min (from 3,096 +/- 204/mm3 to 2,551 +/- 158/mm3, p < 0.05) followed by a rebound neutrophilia. In contrast, plasma histamine level did not change significantly at any time measured. Conversely in the same asthmatics, a rapid rise in plasma histamine level (from 0.29 +/- 0.03 ng/ml at baseline to 0.53 +/- 0.06 ng/ml at 5 min; p < 0.01) was observed after an allergenic challenge (Dermatophagoides pteronyssinus) causing a fall in FEV1 peaking at 10 min (22 +/- 4 percent). Thus, inhaled PAF may induce airway obstruction and neutropenia in asthmatics without any significant change of plasma histamine level. These results indicate that it is unlikely that lung mast cells or basophils degranulate during PAF-induced bronchoconstriction.