The effects of aprotinin on renin release and renal function were evaluated in 24 male essential hypertensive patients, on unrestricted (N = 17) and on chronic low as well as on high Na intake. Aprotinin (1 x 10(6) KIU) or Saline (200 ml) were infused in all patients for 6 hrs. Blood samples were taken for plasma renin activity (PRA) and 6-hr urine collections were obtained for active and inactive kallikrein, Na+, K+, Cl-, Ca++ excretion measurements. In patients on unrestricted sodium diet aprotinin had no effect on BP, glomerular filtration rate, renal plasma flow, urinary sodium and potassium excretion. However an inverse relationship was found between pretreatment urinary sodium excretion and the percent reduction of the latter after aprotinin. A significant reduction in urinary sodium excretion was induced by aprotinin in patients on high sodium intake, whereas no change was observed in the same patients when on low sodium diet. Aprotinin reduced the urinary excretion of active kallikrein by 81% and the active to total kallikrein ratio from 24% to 6%, suggesting that it could interfere with the in-vivo activation of inactive kallikrein. Infusion of aprotinin induced a significant decline in active renin but did not modify inactive renin levels in patients on unrestricted sodium diet as well as in patients on low or high sodium intake. Our data suggest that the inhibition of kallikrein and/or other serine proteases by aprotinin can interfere with renal release of active renin and also support the hypothesis that the renal kallikrein system exerts a regulatory control on sodium excretion in salt replete hypertensives.