Blood pressure has a continuous unimodal distribution in the general population. Individuals with levels elevated above the 80th percentile suffer from hypertension, a major cause of cardiovascular morbidity and mortality. Over 95 percent of hypertensives have no other primary disease and are said to have essential hypertension. Pedigree and twin studies indicate that genetic differences account for a large fraction of the interindividual variation in blood pressure. In 1984 when the study began, no genetic loci had yet been identified that were useful markers for essential hypertension or served as probes into the biological basis for interindividual blood pressure variability. Environmental factors also influence blood pressure; increased dietary intake of sodium chloride, in particular, elevates blood pressure and may contribute to the development of essential hypertension in some people. It was hypothesized that genetically determined alterations in the metabolism of sodium ion occur in a subset of the population that is at increased liability to develop elevated blood pressure upon exposure to the usual range of dietary sodium intake