Such compounds as clonidine, guanabenz and guanfacine, which stimulate central alpha-adrenoceptors, reduce blood pressure predominantly by reducing sympathetic outflow. Notwithstanding, they also tend to reduce renin secretion, aldosterone, and even vasopressin levels. In some clinical settings, inactivation of the renin-angiotensin system may contribute to their antihypertensive effect. In addition, the absence of sodium retention, which appears to be a special feature of guanabenz, may also have a beneficial effect on blood pressure. In contrast, it is unlikely that a decrease in vasopressin secretion has any major influence on blood pressure.