To identify characteristics that may contribute to sodium susceptibility, we conducted, studies in normal subjects who are at risk for hypertension, namely, blacks, subjects older than 40 years of age and first-degree relatives of patients with essential hypertension. All three groups exhibited a decreased natriuretic capacity when compared with control subjects. Blacks and older subjects had consistently low renin values, while the plasma renin activity values of the relatives were greater than those in control subjects. Studies in twins showed that natriuretic capacity and several factors influencing sodium excretion are heritable. When blacks were subjected to extremely high sodium intake, a greater increase in blood pressure developed than in whites. These observations are consistent with an intrinsic renal abnormality in blacks and older subjects resulting in modest volume expansion. In the normotensive relatives of hypertensive patients, the renin system may be responsible for the decreased sodium excretory capacity. These alterations are possibly inherited.