The effect of inhibition of angiotensin-converting enzyme (ACE) on standard cough challenge was investigated in a double-blind, randomised study in sixteen normal volunteers. Captopril (25 mg) or matched placebo was given by mouth 2 h before inhalation of nebulised distilled water, citric acid, and incremental doses of capsaicin (0.5-20 mumol/l). Distilled water and citric acid challenge were not significantly changed by captopril pretreatment. However, captopril significantly shifted the dose-response curve to capsaicin inhalation. The geometric mean dose of capsaicin causing 20 coughs/min was 1.3 mumol/l for captopril and 2.8 mumol/l for placebo pretreatment (p = 0.04). Cough is a recognised side-effect of ACE inhibitors; the observation that cough challenge is changed by these drugs in normal subjects implies a role for ACE in the cough reflex, possibly by metabolism of substrates other than angiotensin I.