In various kinds of hypertension clonidine induced a decrease in urinary catecholamines, plasma renin activity and urinary aldosterone, concommitant with a fall in blood pressure and pulse rate in both short term and chronic studies. Furthermore, clonidine lowered the plasma levels of noradrenaline and adrenaline but a postural increase in upright position still occurred. The capacity to increase renin during salt restriction seemed mainatined. When clonidine was withdrawn all parameters returned to pretreatment levels but in some cases a marked rebound increase in catecholamine production was seen. --During clonidine the increase in catecholamines and renin after insulin induced hypoglycemia was largely abolished. Under basal conditions oral penbutolol induced a decrease of pule rate and blood pressure but no change in plasma or urinary catecholamines. During treatment plasma renin was suppressed at rest and after exercise. A work load, which led to only minor changes in blood catecholamines before treatment, was associated with a marked increase during penbutolol. Medication with penbutolol reduced the response in plasma catecholamines after hypoglycemia and renin activity remained low. Clonidine seems to act mainly by central inhibtion of symapthetic tone. Penbutolol probably acts mainly peripherally but may also have a central effect.