Epidermal growth factor (EGF) is synthesized as a membrane bound precursor in the rat kidney. The precursor seems to be processed by an aprotinin sensitive proteinase. Intravenous infusion of aprotinin reduces the urinary excretion of EGF by 85% and increases the amount of renal EGF. Kidney membranes incubated at 37 degrees C release EGF and this release is inhibited by aprotinin.