Urinary kallikrein excretion was evaluated in 85 normal subjects and in 149 uncomplicated and recently diagnosed essential hypertensive patients. Moreover, the possible interrelationships between urinary kallikrein excretion and age, sex, electrolyte excretion, and plasma renin activity were examined. In patients with essential hypertension, urinary kallikrein excretion was similar to that of normal subjects. In these patients the enzyme was weakly and positively related to urinary potassium and plasma renin activity; no correlation was found with blood pressure, urinary sodium, age, or sex. In normal subjects and in patients with essential hypertension, the variables studied account for only 25% and 17%, respectively, of the variability of urinary kallikrein excretion. We conclude that the relatively short duration of hypertension in our patients may explain the unaltered values of urinary kallikrein excretion with respect to controls.