1. Fifty patients with uncomplicated essential hypertension were hospitalized and put on a diet containing 55 mmol of sodium daily.
2. Blood pressure was measured at 2 h intervals and averaged for each day. On the first, second, third and seventh days blood samples were drawn for measurement of renin and catecholamines. On the basis of levels on the seventh day patients were divided into a low-renin, normal-renin and a high-renin group.
3. When on balance on their sodium-restricted diet low-renin patients had significantly higher blood pressure, but similar catecholamine levels to the other groups. Cumulative sodium loss on the seventh day was significantly greater in low-renin patients than in the others.
4. It is concluded that low-renin patients exhibit enhanced sensitivity to the effect of sodium. The low-renin state seems to be associated with more severe hypertension, rather than with a disturbance in sympathetic activity.