Kinins and prostaglandins of the E series (PGE) have the capacity to influence renal hemodynamic and excretory events and may interact intrarenally so as to reinforce one another. Thus, in the isolated Krebs-perfused rabbit kidney we showed that addition of either bradykinin or kininogen to the perfusing fluid augments the release of a PGE-like substance and that aprotinin, a kallikrein inhibitor, reduces the release of prostaglandins evoked by kininogen but not by bradykinin. Moreover, we have observed that deoxycorticosterone, an agent which increases urinary kallikrein, enhances the urinary excretion of PGE-like substance, and that this effect is prevented by simultaneous treatment with aprotinin. These observations and our demonstration that enhanced intrarenal activity of the kallikrein-kinin system, consequent to kininase II inhibition, is associated with renal vasodilation, diuresis, and natriuresis, suggest that a coupling of kinins and prostaglandins intrarenally may be directed towards the facilitation of salt-water excretion. The interdigitation of prostaglandins and the kallikrein-kinin system may thereby constitute the essential operation of a regulatory system in which the complementary actions of these hormones antagonize the sodium retaining effect of mineralocorticoids in those states in which salt-water balance is positive.