In 26 patients with mild-to-moderate hypertension, 80.8% of whom had a history of concomitant diseases, the effect of moxonidine (0.2 mg b.i.d.) on the 24-h ambulatory blood pressure profile (ABPM) was compared with captopril (25 mg b.i.d.) in a double-blind, parallel-group study. After 4 weeks of treatment with placebo, ABPM was performed and the patients were treated with moxonidine (n = 14) or captopril (n = 12) for a further 4 weeks. ABPM was then repeated. Both moxonidine and captopril reduced systolic and diastolic blood pressure sufficiently and to the same extent. Mean 24-h pulse rate and standard laboratory parameters were not changed by active treatment. After drug withdrawal for 5 days, sitting blood pressure did not differ from baseline values in both groups. Serious adverse events did not occur, the most frequent complaints were nausea (2 of 14 patients receiving moxonidine) and dizziness (3 of 12 patients receiving captopril). We concluded that the blood pressure-lowering effects of moxonidine (0.2 mg b.i.d.) and captopril (25 mg b.i.d.) are comparable in patients with mild-to-moderate hypertension.