Several studies have shown an inverse relation between blood pressure and plasma aldosterone levels. Since blood pressure is in part genetically regulated, we looked for evidence that genetic factors might also affect aldosterone production. The nocturnal urinary excretion rate was used to estimate aldosterone production, and electrolyte excretion rates were used to estimate sodium and potassium intakes. Studies were carried out in monozygotic (MZ) (n = 37 pairs) and dizygotic (DZ) (n = 26 pairs) twins, aged 6-17 years. Both groups of twins were white. The intraclass correlation coefficient for aldosterone excretion was 0.686 (p = 0.0001) for MZ twins, and 0.290 (p = 0.079) for DZ twins, indicating high heritability for the aldosterone excretion rate. In a second study, we looked for a racial effect on the genetic regulation of aldosterone excretion. Siblings from both black and white families (72 black siblings and 157 white siblings) were selected from an ongoing longitudinal study. Mean values for nocturnal aldosterone excretion, rates measured every 6 months over 1.5-3.5 years, were used in the analysis. The intraclass correlation coefficient for aldosterone excretion, adjusted for sodium and potassium excretion, was 0.510 (p = 0.001) for black siblings and 0.087 (p = 0.228) for white siblings, indicating a strong familial aggregation for aldosterone excretion in black children. In conclusion, studies in twins showed that regulation of urinary aldosterone excretion in children is determined partially by genetic factors. A familial component affecting the aldosterone excretion rate appears to be much stronger in blacks than in whites.