After improvement of technical equipment continuous ambulatory blood pressure monitoring is more and more used in the diagnosis of hypertension. New fully automatic systems permit a reliable registration and evaluation of 24-h blood pressure profiles. Typical circadian rhythmics of blood pressure, independent of a variability with different grades of activity, can be demonstrated in normotensive persons and also in patients with essential hypertension. Patients with secondary forms of hypertension show a nivellation or offset of circadian blood pressure rhythmics. A study was performed to examine the antihypertensive efficacy of the calcium antagonist Nitrendipine, the beta 1-adrenoceptor-selective blocker Metoprolol, the beta-blocker with intrinsic activity Mepindolol and the angiotensin converting enzyme inhibitor Enalapril in patients with mild to moderate hypertension over a period of 6 month. Continuous ambulatory blood pressure monitoring was performed before and after 6 month of therapy. 98 of 299 included patients broke off therapy, 47 of those because of side effects. Hydrochlorothiazide was given additionally if the antihypertensive effect of monotherapy was not sufficient after a period of 4 weeks. Morning blood pressure controls at the end of the treatment period showed normotensive values in all groups without significant differences between the groups before and at the end of the treatment period. The number of prescriptions of diuretics necessary to achieve normotension differed between the four treatment groups: Nitrendipine (n = 5), Metoprolol (n = 7), Mepindolol (n = 14), Enalapril (n = 20). In contrast to the morning blood pressure values the continuous 24-h blood pressure monitoring demonstrated significant differences between the therapy groups. Metoprolol turned out as most effective in lowering blood pressure and in reducing the number of systolic blood pressure peaks above 180 mmHg, but on the other hand showed the highest incidence of relative hypotension (less than 100 mmHg systolic, less than 80 mmHg diastolic). Mepindolol demonstrated a significant lower efficacy. In the Nitrendipin group least of all prescriptions of diuretics were necessary and the lowest number of hypotensive systolic blood pressure values occurred. Enalapril showed the most significant reduction of diastolic values above 100 mmHg and the lowest number of diastolic values below 80 mmHg, but the highest number of prescription of diuretics was necessary in the Enalapril group. In none of the four therapy groups a neutralisation of circadian blood pressure rhythmics was demonstrable.