In a previous study we have shown that inhibition of the endogenous neuropeptide-degrading enzyme, neutral endopeptidase (NEP), potentiates airway narrowing to neurokinin A (NKA) in normal humans in vivo. In the present study, we tested the hypothesis that hyperresponsiveness to NKA in asthma is caused by a reduction in endogenous NEP activity. To that end, we used the NEP inhibitor, thiorphan, or placebo as inhaled pretreatment to NKA challenge in eight atopic asthmatic men, who were controlled by on-demand usage of β2-agonists alone. The dose of thiorphan pretreatment was obtained from pilot experiments in which 0.5 ml of a 2.5-mg/ml concentration appeared to be the maximally effective nebulized dose. Doseresponse curves to inhaled NKA (1 to 125 μg/ml, 0.5 ml/dose) were recorded on 2 randomized days 1 wk apart, in a cross-over study. To detect any effects of thiorphan on bronchoconstriction per se, we also investigated the effect of thiorphan or placebo on the dose-response curve to inhaled methacholine in a separate set of experiments. The response was measured by FEV1 and by partial expiratory flow-volume curves (V̇40p). The position of the dose-response curves was expressed as the concentration causing a 20% fall in FEV1 (PC20FEV1 or a 40% fall in V̇40p (PC40V̇40p). Baseline FEV1 and V̇40p were not affected by either pretreatment (p > 0.06). PC20FEV1 and PC40V̇40p to NKA were significantly lower after thiorphan pretreatment as compared with placebo (mean difference ± SEM: 2.3 ± 0.6 and 1.6 ± 0.5 doubling dose, respectively; p < 0.015). There was no significant difference in PC20FEV1 or PC40V̇40p to methacholine between thiorphan and placebo treatment (p > 0.70). We conclude that thiorphan enhances airway narrowing to neurokinin A in asthmatic subjects without affecting bronchoconstriction to methacholine. These results suggest that hyperresponsiveness to NKA in clinically stable asthmatics is not caused by a reduction in endogenous NEP activity.