1. The levels of plasma renin activity or concentration and plasma angiotensin were similar in normotensive subjects and in untreated or treated essential hypertensives.
2. There was a close relationship between plasma renin activity and plasma renin concentration and between these and plasma angiotensin. This suggests that there were no abnormalities in renin substrate or renin kinetics in human essential hypertension.
3. Ganglion blockade did not change the plasma renin levels but α-adrenergic blockade caused a significant rise in plasma catecholamines and plasma renin without change in blood pressure.
4. Both acute and chronic β-adrenergic blockade were associated with a fall in blood pressure and small but significant falls in plasma renin levels.