The evolution of blood pressure, heart rate, epinephrine, norepinephrine, angiotensin II and plasma renin activity has been studied in 10 patients with essential arterial hypertension before and during a two months period of treatment with guanfacine, a new centrally acting hypotensive drug. Guanfacine was proven effective in lowering both systolic and diastolic supine and standing blood pressure. A decrease in supine and standing norepinephrine plasma concentrations and plasma renin activity was observed. No change was seen in epinephrine or angiotensin II. The fall in supine blood pressure observed during the treatment period was positively correlated with the change in norepinephrine.