Endogenously released cyclooxygenase products modulate the bronchoconstrictor response to various stimuli in asthma. Little is known of the change in airway responsiveness to neurokinin A (NKA) after cyclooxygenase blockade. In this randomized, double-blind, placebo-controlled study, we have investigated the effect of the potent cyclooxygenase inhibitor, lysine acetylsalicylate (L-ASA) administered by inhalation, on the bronchoconstrictor response both to neurokinin A (NKA) and methacholine in nine asthmatic subjects. Subjects attended the laboratory on four separate occasions to receive nebulized L-ASA (solution of 90 mg.mL-1) or matched placebo (glycine, solution of 30 mg.mL-1) 15 min prior to bronchial challenge with NKA or methacholine, in a randomized, double-blind order. Changes in airway calibre were followed as forced expiratory volume in one second (FEV1) and agonist responsiveness, expressed as the provocative concentration causing a 20% fall in FEV1 from baseline (PC20). L-ASA elicited a significant fall in FEV1 from baseline. When compared with placebo, inhaled L-ASA reduced the airway responsiveness to NKA in 8 of the 9 subjects studied, the geometric mean (range) values for PC20 NKA increasing significantly from 153.2 (52.0-258.9) to 303.1 (83.4-668.5) micrograms.mL-1 after placebo and L-ASA, respectively. However, no significant change in airway responsiveness to methacholine was recorded after L-ASA, their geometric mean (range) PC20 values being 1.60 (0.17-9.59) and 1.53 (0.09-14.01) mg.mL-1 after placebo and L-ASA, respectively. The small decrease in airway responsiveness to neurokinin A after administration of lysine acetylsalicylate by inhalation suggests that endogenous prostaglandins may play a contributory protective role in the airway response to neurokinin A in human asthma.