To assess the effect of age on the circadian blood pressure (BP) after an alpha,beta-adrenergic blocker, the ambulatory BP was measured before and after arotinolol chloride administration in nine younger (mean age 49.1 years) and 14 older (72.1 years) patients with essential hypertension. After a 4-week control period, arotinolol chloride was administered twice daily (08:00 and 20:00 hours) for 8 weeks and the ambulatory BP was measured non-invasively at the end of the control and treatment period. Arotinolol significantly reduced the daytime systolic BP from 152.3 to 140.9 mmHg (P < 0.05) and night-time systolic BP from 137.3 to 122.3 mmHg (P < 0.01) in the younger hypertensive patients. In contrast, in the older group, the night-time systolic BP did not show a significant change, although the daytime systolic BP was significantly reduced from 155.0 to 142.2 mmHg (P < 0.02). Diastolic BP in both groups was significantly reduced by arotinolol during the day and night. Night-time reduction of BP was significantly less in the older group (-8.6 vs -15.1 mmHg for the systolic pressure P < 0.01; -5.8 vs -9.8 mmHg for the diastolic pressure P < 0.01).