A genetic analysis of twins at school was undertaken using as variables urinary concentrations of kallikrein, catecholamines, sodium and potassium which have been demonstrated to be associated with blood pressure levels. In addition to these variables, urinary concentrations of urea nitrogen and inorganic sulfate sulfur which are indices of protein intake were investigated. 35 pairs of monozygotic twins and 19 pairs of dizygotic twins aged from 6 to 14 years were examined. Variance and correlation tests for genetic analysis indicated that in school children, hereditary factors play a role in the control of urinary potassium, sodium and kallikrein excretion. However, with regard to the urinary excretion of catecholamines, urea nitrogen and inorganic sulfate sulfur, hereditary control is not so apparent.