When the study began in 1984, the objective was to determine whether genetic alterations in pathways of sodium transport in the red blood cells of children could predict their risk of developing primary hypertension in adulthood. Approximately 600 children between 7 and 18 years of age were randomly selected as index cases for family studies. Each participating member of the pedigrees studied underwent venipuncture for withdrawal of whole blood. A medical history was obtained from each family member visiting the clinic and a physical examination was conducted. Existing medical records for all living and deceased adult relatives in the 600 pedigrees were reviewed. In addition, 40 normotensive adult members of 300 pedigrees underwent metabolic balance and renal clearance studies, as well as ambulatory blood pressure recordings. Segregation analysis of pedigree data was carried out to determine the most likely mode for genetic transmission of the sodium-lithium counter-transport phenotype. Baseline genetic information about sodium-lithium was related to the prevalence of primary hypertension in the families of index children. Additional phenotypes involved in blood pressure regulation were also studied including renal proximal tubular sodium reabsorption, atrial natriuretic peptide, sodium-potassium adenosinetriphosphate pump, and sodium-potassium cotransport.