The present study on the rat shows that i.v. administration of the proteinase inhibitor aprotinin reduces the urinary output of immunoreactive epidermal growth factor (EGF) while the amount of immunoreactive EGF in the kidneys is increased. This indicates that the EGF-precursor in the rat kidney in vivo is processed by an aprotinin inhibitable proteinase. EGF is produced in the kidneys as a precursor with a molecular weight of approximately 130 kDa. In rat urine, nanomolar amounts of 6 kDa EGF are excreted per 24 h together with small amounts of high molecular weight forms of EGF. During i.v. administration of aprotinin the median urinary output of immunoreactive EGF is reduced to 15% of the excretion of control rats (23 pmol/2 h versus 157 pmol/2 h, P less than 0.001). Especially the excretion of 6 kDa EGF is reduced (median excretion 12 pmol/2 h versus 134 pmol/2 h, P less than 0.001). The amount of immunoreactive EGF in the kidney tissue is increased after aprotinin administration (median amount 0.11 pmol EGF/mg protein versus less than 0.04 pmol EGF/mg protein, P less than 0.001). Neither the creatinine clearance, the total urinary protein output, nor the volume of urine produced was affected by aprotinin.