We compared the antihypertensive effects of acebutolol and metoprolol during 2-4 weeks of treatment in patients with mild to moderate essential hypertension. Acebutolol (n = 12) significantly decreased conventionally measured blood pressure from 173/100 mmHg to 148/86 mmHg (p less than 0.005), and metoprolol (n = 11) decreased it from 164/106 mmHg to 138/87 mmHg (p less than 0.01). Based on data derived from automated 24-hour ambulatory blood pressure monitoring, both drugs significantly decreased the blood pressure in the early morning (5:00-10:00). Moreover, in the metoprolol group, there were significant falls in day-time blood pressure (7:30-19:30) and night-time blood pressure (23:00-7:00). In contrast, acebutolol showed significant antihypertensive effect on day-time blood pressure, but not effect on night-time blood pressure. The study confirmed the efficacy and character of metoprolol and acebutolol. We must choose an effective beta-blocker when using an automated 24-hour ambulatory blood pressure monitoring system to get adequate blood pressure reduction for the whole 24 hours.