When administered by inhalation, bradykinin provokes dose-related bronchoconstriction in asthmatic subjects by a mechanism believed to involve activation of sensory nerve endings. However, little is known of the change in airway responsiveness to bradykinin after cyclo-oxygenase blockade. The aim of the present study was to investigate the effect of the potent cyclo-oxygenase inhibitor, lysine acetylsalicylate (L-ASA), administered by inhalation, on bradykinin-induced bronchoconstriction in a group of 12 asthmatic subjects. The subjects attended the laboratory on four separate occasions to receive nebulized L-ASA (solution of 90 mg x mL(-1)) or matched placebo (glycine, solution of 30 mg x mL(-1)) 15 min prior to bronchoprovocation tests with bradykinin and methacholine in a randomized, double-blind order with at least a 5 day interval. Changes in airway calibre were followed as forced expiratory volume in one second (FEV1), and responsiveness to agonists was expressed as the provocative concentration causing a 20% fall in FEV1 from baseline (PC20). Administration both of L-ASA and glycine solution caused a small but significant acute fall in FEV1 from baseline, with gradual recovery within 20 min. When compared to placebo, inhaled L-ASA reduced the airway responsiveness to bradykinin in 11 of the 12 subjects studied, the geometric mean (range) values for PC20 bradykinin increasing significantly (p<0.001) by 1.7 doubling dose from 0.55 (0.11-5.05) to 1.72 (0.26-6.05) mg x mL(-1) after placebo and L-ASA, respectively. No significant change in airway responsiveness to methacholine was recorded after L-ASA. It is concluded that administration of lysine acetylsalicylate by inhalation protects the asthmatic airways against bradykinin-induced bronchoconstriction, thus suggesting that endogenous prostaglandins may play a contributory role in the bronchoconstriction to kinins in human asthma.