In the context of a controlled antihypertensive drug trial, blood pressure readings obtained by the physician in his office were compared with ambulatory blood pressures recorded with the semi-automatic Remler device. The beta-adrenoceptor blocking agent timolol or methyldopa were administered in double-blind fashion to 30 patients with uncomplicated essential hypertension. All exhibited a diastolic office blood pressure of greater than 95 mm Hg at the end of a 4-week placebo period. All patients then received a combination of hydrochlorothiazide (25 mg/d) and amiloride (2.5 mg/d). After 4 weeks of diuretic therapy, timolol (10 mg/d, n = 14) or methyldopa (250 mg/d, n = 16) were added randomly for 6 weeks. The dose of all antihypertensive agents was doubled after 2 weeks of therapy with diuretics combined with timolol (n = 7) or methyldopa (n = 16) because of the persistence of diastolic blood pressure levels of greater than 90 mm Hg at the office. When assessed in the office, the antihypertensive effect of timolol and methyldopa was similar. During ambulatory blood pressure monitoring, however, pressure levels tended to be lower in the patients given timolol (p less than 0.05 for the diastolic). These data show that ambulatory blood pressure monitoring may be very useful in comparing the antihypertensive efficacy of different drugs.