The investigators believed that the mechanisms responsible for sodium-dependent blood pressure were only indirectly linked to race, sex, and age. Their research was consistent with the hypothesis that sodium-dependent blood pressure control resulted from decreased responsiveness of the renal and adrenal blood pressure regulatory systems. This produced abnormal daily blood pressure patterns, with reduced fluctuation resulting in extended periods of increased blood pressure. The increased cardiovascular load led to the early development of cardiovascular and renal disease, including hypertension. They tested this hypothesis by identifying 'high risk' and 'normal risk' subjects based on renal responses to sodium restriction. The subjects were equal numbers of healthy males and females, Blacks and whites, between the ages of 55-70 years. Using an innovative approach, they then examined the influence of risk status on 24-hour patterns of blood pressure, hormonal activity, and sodium handling. Finally, they determined the clinical significance of the profiles by examining changes in cardiac and renal status at a two-year follow-up.