Fourty-six men and 6 women aged 45 years and having arterial hypertension newly diagnosed at routine medical examinations were given out-patient antihypertensive treatment with prazosin, prazosin + hydrochlorothiazide, or prazosin + hydrochlorothiazide + clonidine. The mean values of blood pressure after the 3-week placebo period were 157/109 mmHg in the supine and 160/115 mmHg in the standing position. Treatment with prazosin (1--2 mg t.i.d.) produced normotension in 4/52 patients only, yet supine diastolic blood pressure and standing blood pressure were significantly lowered within 9 weeks. The addition of hydrochlorothiazide (25 mg daily) for 3 weeks to the regimen led to normotension in 12/46 patients. The remaining 34 patients still having an average supine blood pressure of 152/106 mmHg after prazosin + hydrochlorothiazide, responded well to low doses of clonidine added for 6 weeks to the treatment. Only 7 patients having initially high blood pressure still had a diastolic blood pressure greater than or equal to 100 mmHg at the end of the trial. The subjective side-effects were frequent but mild being roughly similar during placebo and active drug periods, except that fatigue and dry mouth due to clonidine were common, yet tolerable. No "first tablet reactions" to low inital doses of prazosin were found.