Plasma Na+ concentration is regulated within narrow limits. Yet, substantial interindividual differences exist even in the normal range. The determinants of these differences are not well understood. We therefore investigated the clinical and neurohumoral associates of plasma Na+. We studied 2,364 men (age: 48 +/- 12 yr) and 2,710 women (age: 47 +/- 12 yr) from the prospective Prevention of Renal and Vascular End-Stage Disease (PREVEND) cohort study. In the present study, we investigated the neurohumoral factors NH2-terminal prohormone of brain natriuretic peptide (NT-proBNP) and aldosterone as volume markers and copeptin as a marker for osmoregulation. Clinical associating variables of plasma Na+ were age, sex, and plasma glucose. Furthermore, plasma Na+ levels were associated with log(2) copeptin (men: standardized beta = 0.18, P <0.001; women: standardized beta = 0.17, P <0.001), log(2) NT-proBNP (men: standardized beta = 0.07, P = 0.008; women: standardized beta = 0.12, P <0.001), and log(2) aldosterone (men: standardized beta = -0.06, P = 0.005; women: standardized beta = -0.09, P <0.001). Copeptin and NT-proBNP showed an interaction in their association with plasma Na+. Thus, our data 1) support that osmo-regulation, as estimated from copeptin levels, is a main associate of plasma Na+; 2) show a consistent association with volume markers, with higher NT-proBNP and lower aldosterone in individuals with higher plasma Na+; and 3) show that the interaction between copeptin and NT-proBNP illustrates that osmoregulation and volume regulation act in concert in the regulation of plasma Na+.