Neutral endopeptidase (NEP) is found in many tissues in man, including the lung. Metabolism by NEP is one of the main mechanisms for the clearance of atrial natriuretic peptide (ANP), a hormone that causes bronchodilation and reduces nonspecific bronchial reactivity in man. Candoxatril, an oral NEP inhibitor has been shown to elevate circulating ANP levels. We have sought to determine whether the administration of candoxatril will alter bronchomotor tone (forced expiratory volume in one second (FEV1)) and histamine reactivity. Ten male asthmatic patients with stable asthma were enrolled (mean (SD) age 32 (10) yrs; FEV1 92 (11)% predicted) in a randomized, double-blind, placebo-controlled study. On each study day, after baseline spirometry, patients received 200 mg of candoxatril or placebo. Spirometry was repeated at half hourly intervals. After 2 h a histamine inhalation test was performed. There was no significant difference in FEV1 values at baseline or at 2 h post-dosing between active and placebo study days, with mean (SEM) FEV1 at baseline and 2 h of 3.71 (0.29) l and 3.85 (0.29) l on the placebo day, and 3.89 (0.27) l and 4.05 (0.82) l on the active day, respectively. The geometric mean (range) provocative concentration of histamine producing a 20% fall in FEV1 (PC20) on the placebo day and active day did not differ significantly, being 1.17 (0.25-25.8) and 0.93 (0.13-32) mg.ml-1, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)