The aim of this study was to investigate the impact of sex and puberty stage on circadian changes in sodium excretion, sodium regulating hormones and hemodynamics. 39 healthy volunteers (9 pre-puberty boys, 10 pre-puberty girls, 10 puberty boys and 10 puberty girls) were included. They all underwent a 24-hour circadian in-patient study under standardized conditions regarding activity, diet and fluid intake. Blood samples were drawn every four hours and the urine was collected in fractions. Blood pressure and heart rate were non-invasively monitored. Atrial natriuretic peptide (ANP), angiotensin II, aldosterone, and renin were measured in blood. Results: Children in puberty had lower plasma levels of renin (p<0.05) and angiotensin II (p<0.05), and a 26 % reduction in filtered sodium without changes in sodium excretion when compared to pre-puberty children. A circadian rhythm in sodium excretion, the renin-angiotensin system, ANP and blood pressure was found with a midnight ANP peak (p<0.001), a night-time decrease in hemodynamic parameters(p<0.001), an increase in plasma renin (p<0.001) and angiotensin II (p<0.001) and a decrease in sodium excretion (p<0.001) mainly on the basis of increased sodium reabsorption (p<0.001). The timing of the changes did not depend on sex or puberty group. Conclusion: There is a circadian rhythm of sodium excretion and sodium regulation in 7-15year old children. This rhythm is similar in boys and girls. As an important new finding, puberty changes the plasma levels of renin and angiotensin II without changing the amount of sodium excreted or the day to night sodium excretion ratio.