The acute effects of doxazosin, a new selective alpha-1 adrenergic blocking agent, on blood pressure, heart rate, and on some circulating vasoactive hormones, such as plasma renin activity, catecholamines, serotonin and endothelin-1 in ten healthy normotensive volunteers and eight moderate-severe essential hypertensives were studied. Two milligrams doxazosin was administered orally in randomised fashion as compared to placebo and the acute effects were evaluated one half to four hours later. Doxazosin did not reduce blood pressure and did not increase heart rate in normotensive subjects as compared to placebo. A significant decrease in blood pressure was found in hypertensives after doxazosin (p < 0.01), without change in heart rate. Simultaneously, doxazosin did not modify plasma renin activity, catecholamines, serotonin and endothelin-1 concentrations both in normotensive and hypertensive subjects when compared with placebo administration. Thus, it appears that doxazosin does not influence local and systemic vasopressor hormones involving in the regulation of blood pressure. In conclusion, the lack of effects on plasma vasoactive factors confirms the selective alpha-1 postsynaptic antagonism of doxazosin and an effective antihypertensive action in the treatment of essential hypertension.