Recent evidences suggest that the sympathetic nervous system plays a major role in the pathogenesis and maintenance of several clinical forms of hypertensive disease. The clonidine suppression test allows us to uncover the subtle changes in neuronal noradrenaline release in essential hypertension. To prove this possibility, we selected 16 patients: 7 borderline hypertensives (BH) (4 males and 3 females), mean age: 24 years; and 9 established hypertensives (EH) (6 males and 3 females), mean age: 25 years. In both groups, the heart rate (HR) and blood pressure (BP) were registered every 30 min and by intravenous catheter plasma catecholamines (PCA) and plasma renin activity (PRA) were measured before and after (180 and 240 min) one oral dosis of clonidine (300 mcg) had been administered. Patients remained in clinostatism for 180 and and orthostatism for 60 min. HR and BP diminished in BH and EH after clonidine during clinostatism in comparison to preclonidine period. PCA also showed reduction in 180 min with respect to the basaline period. PRA did not present change in 180 min in either groups in relation to time O. During orthostatism, HR increased in both groups with respect to the baseline period. BP remained low in relation to baseline time. PCA were increased with respect to preclonidine period. PRA showed a slight tendency to increase in relation to time O in both groups. CAP baseline level are not reliable indexes of the role of the sympathetic tone in maintenance of high BP values in hypertensive disease.(ABSTRACT TRUNCATED AT 250 WORDS)