Biochemical alterations that have been correlated with elevated blood pressure have not received extensive epidemiologic study because of the technical difficulties involved. Because the excretion of urinary kallikrein is reduced significantly in adult hypertensives, we have studied urinary kallikrein in a cohort of children in whom familial aggregation of blood pressure has been demonstrated. Casual specimens of urine were obtained in household surveys, and urinary concentration of kallikrein was determined in 601 children aged 5-18 years. The children were from 163 families, whose members also had their blood pressures measured. Familial aggregation of urinary kallikrein concentration was found by analysis of variance (F=3.45, p less than 0.001) even in these casual specimens and was demonstrable for black and white children analyzed separately. Urinary kallikrein concentration was significantly lower in black children than in white children (p less than 0.001) and was positively correlated with urinary creatinine and urinary potassium and inversely related to urinary sodium concentrations. Urinary kallikrein concentration also was being altered by season (being lowest in the summer) and by time of day (being highest in the morning). Families with the lowest mean kallikrein concentrations tended to have higher blood pressures than did families with the highest mean kallikrein concentrations, although the effect is small and subject to many variables.