1. The changes in plasma volume, haemodynamic variables, plasma renin activity and plasma aldosterone were studied in forty-one hypertensive patients after administration of adrenergic-blocking agents. Four drugs were used: alpha-methyldopa (fourteen patients), guanethidine (ten patients), clonidine (nine patients) and reserpine (eight patients). Drugs were administered orally during 7 days' hospitalization on a normal sodium diet (110 mmol/day). 2. The four drugs had similar effects: a significant decrease in blood pressure, a significant increase in plasma volume and no change in stroke volume. 3. With alpha-methyldopa and guanethidine, heart rate, plasma renin activity and plasma aldosterone were unchanged. 4. With reserpine and clonidine, heart rate and plasma renin activity were significantly decreased, whereas plasma aldosterone did not change significantly. 5. This study suggests that the decrease in plasma renin activity was related to the lowering of the heart rate rather than to sodium retention and that adrenergic-blocking agents can impair the normal relationship between stroke index and plasma volume, between plasma volume and plasma renin activity, and between plasma renin activity and plasma aldosterone.