OBJECTIVE: To determine the effects of ibopamine 100 mg three times daily compared with captopril 25 mg three times daily on exercise capacity in patients with chronic heart failure. DESIGN: A randomised, double blind, parallel group comparison of the addition of ibopamine versus captopril during a period of 24 weeks. SETTING: 26 outpatient cardiology clinics in seven European countries. PATIENTS: 266 patients, with mild to moderate chronic heart failure (New York Heart Association (NYHA) functional class II, 81% and III, 19%) and evidence of an enlarged left ventricle. Patients received concomitant treatment with diuretics and/or digitalis. MAIN OUTCOME MEASURE: Exercise duration after 24 weeks of treatment, compared with baseline. RESULTS: Mean (SD) ejection fraction was 29 (8)% and the baseline exercise duration in the captopril and ibopamine groups 665 (160) and 675 (174) seconds, respectively. At the end of the study, exercise duration had improved in both groups, by 29 seconds in the ibopamine group (P < 0.01), and by 24 seconds in the captopril group (P < 0.05). There was no difference between groups (P = 0.69, 95% confidence interval -22 to 33). NYHA class, signs and symptoms score, and dyspnoea and fatigue index improved equally in both groups. The total number of adverse events was the same in both treatment groups, but gastrointestinal complaints occurred more often in the ibopamine group. The number of patients with premature withdrawals was no different. CONCLUSIONS: No difference was detected between the effect of captopril and ibopamine on exercise time in patients with mild to moderate heart failure during a treatment period of 24 weeks.