The antihypertensive effect of Minizide(0.5mg prazosin hydrochloride and 0.25mg polythiazide per tablet)was evaluated in 30 patients with essential hypertension with a dosage of one tablet a day for four weeks. The results were as follows : 1) Of 30 patients treated with Minizide, 21 patients(70.0%) achieved normalization of blood pressure and 5 patients(16.7%) had good response. 2) Mean drop in systolic and diastolic pressure were 29.0mmHg and 15.9mmHg respectively in supine position(P<0.005). 3) Mean drop in systolic and diastolic pressure were 28.5mmHg and 16.2mmhg respectively in sitting position(P<0.005). 4) In 14 patients(46.7%) among 30 patients treated with Minizide adverse reactions developed. The most frequent one was postural dizziness(23.3%). 5) Most adverse reactions were transient and tolerable. However, in three patients(two patients with postural dizziness and one patient with dry mouth) the medication was stopped due to adverse reactions.