1. In eight hypertensive patients with ACE inhibitor-induced cough the resolution of the cough was examined in a prospective observational study over 4 weeks duration. Resolution of cough was measured by visual analogue scales and questionnaire at baseline and days 3, 7, 14 and 28. In addition changes in cough sensitivity to inhaled capsaicin, and skin responses to bradykinin and substance-P were measured at the same time points. 2. All patients recorded significant subjective improvement in cough questionnaire scores for severity and night time waking, and by visual analogue scales for severity and frequency of cough (all P < 0.0005 for trend from day 0-28). Significant changes in subjective measures were recorded by 3 to 7 days for most measures, but further reductions were observed up to day 28 (all P < 0.01 day 28 vs day 0). 3. The sensitivity to inhaled capsaicin fell over the 28 days of study after stopping enalapril. The potency of capsaicin relative to day 0 was reduced to 0.25 (95% CI 0.07-0.87) by day 14, and to 0.20 (95% CI 0.06-0.67) by 28 days. 4. After stopping enalapril there was a highly significant reduction in wheal area produced by intradermal bradykinin, with the majority of the effect seen by day 3 (P < 0.0005). The wheal area to intradermal substance-P also declined with time after stopping enalapril, but significant changes were not observed until 14 days (P < 0.01). 5. Multiple regression analysis of the rates of decline for the subjective and objective measures of cough showed significant associations between the response to inhaled capsaicin and the VAS scores for severity of cough (P = 0.005) and frequency of cough (P = 0.011). Capsaicin response was not related significantly to the severity of cough measured by self-administered questionnaire. 6. There was a significant association between bradykinin response and VAS scores for frequency of cough (P < 0.04) and severity of cough (P < 0.05), but not with cough by questionnaire or the capsaicin response. The response to substance-P did not relate significantly to any of the measures of cough. 7. Cough caused by enalapril improved markedly by 14 days but took up to 28 days to resolve. It was associated with increased sensitivity to inhaled capsaicin which decreased over 28 days, and which paralleled changes in subjective cough scores.