1. The anti-hypertensive effect of atenolol was greater than that of bendrofluazide. 2. The change in systolic blood pressure caused by bendrofluazide showed a significant inverse correlation iwth the plasma renin concentration (r=-0-47; P less than 0-01). 3. The correlation between the change in systolic blood pressure caused by atenolol and the plasma renin concentration was not significant (r =0-28; 0-1 greater than P greater than 0-05). 4. Plasma renin concentration was unaffected by atenolol.