1. Prazosin decreases blood pressure in normotensive, renal hypertensive and spontaneous hypertensive rats. The effect is greatest in the last-named.
2. In spontaneously hypertensive rats the decrease in pressure is associated with a decrease in heart rate.
3. In hypertensive patients prazosin decreases blood pressure by decreasing total peripheral resistance with minor effects on cardiac output.
4. Prazosin is effective in the long-term therapy of hypertensive patients, alone and in combination with a diuretic. The effect on blood pressure is the same in the supine and standing position.