Abstract—Impaired stress-induced pressure natriuresis, ie, an inadequate compensatory increase in urinary sodium excretion (U_NaV) in response to a stress-induced blood pressure increase, may lead to the premature development of essential hypertension. To assess the heritability of baseline U_NaV, stress U_NaV, and the U_NaV response to stress (U_NaV=stress U_NaV- baseline U_NaV), we studied 396 black and 494 white twins, including monozygotic and dizygotic twins of the same as well as the opposite sex (mean age: 17.6±3.3 years; range: 11.9 to 30.0 years). Bivariate genetic model fitting was performed to examine the extent to which genetic and environmental factors are common or specific to baseline and stress U_NaV. Heritability estimates for U_NaV can be derived from these bivariate models. All of the bivariate analyses were performed separately in whites and blacks, because univariate models for baseline U_NaV showed significant ethnic differences in heritability estimates. Best-fitting models showed that the heritability of stress U_NaV was 0.42 in whites and 0.58 in blacks. Only 15% and 11% of the total variance could be attributed to genetic factors common to baseline and stress U_NaV in whites and blacks, respectively. After removal of all of the shared influences with baseline U_NaV, heritabilities for stress U_NaV were 0.32 in whites and 0.57 in blacks. Heritability estimates for U_NaV were 0.36 in whites and 0.39 in blacks. In summary, this study establishes U_NaV and stress U_NaV as heritable phenotypes that may be used to study the genetic etiology of early hypertension development.