1. The effects of oral administration of 4·5 mg of prazosin/day on blood pressure and on plasma renin activity were assessed in patients with essential hypertension and in healthy normotensive volunteer subjects.

2. Mean blood pressure in the hypertensive group fell significantly within 2 weeks of treatment, whereas heart rate was little affected. None of the parameters measured in the normotensive subjects during treatment with prazosin revealed significant changes.

3. Sub-classification of the hypertensive patients into low-, normal- and high-renin categories revealed significant decreases of renin in all six patients with high renin- and in four out of 12 patients with normal renin- but not in the two patients with low renin-essential hypertension.

4. The results indicate that prazosin exerts its hypotensive action and suppresses plasma renin activity particularly in patients with high renin-essential hypertension. These may be considered to have increased sympathetic drive.

5. Conceivably, the blood pressure-lowering effect of prazosin is effected by peripheral vasodilatation and also by a decreased venous return to the heart.

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