Epidermal growth factor (EGF), a protein comprising 53 amino acids, is derived from a precursor of 1,217 amino acids that includes at least seven EGF-like sequences^1,2. EGF has diverse biological activities (see refs 3–6): it is a potent mitogen for many tissue culture cells, inhibits gastric acid secretion from the intestinal mucosa and promotes healing of the corneal epithelium. EGF given to fetal animals accelerates several developmental processes including palate formation, incisor eruption, eyelid opening and lung maturation. However, the physiological roles of EGF in vivo are unknown. The presence of high-affinity receptors in many fetal⁷ and adult tissues⁴ suggests that EGF is involved in normal cellular functions. Immunocytochemical studies have revealed the presence of EGF in mouse⁸ and human Submaxillary glands^9,10, rat brain¹¹ and human intestine^9,10. The low levels of EGF in extracts from many tissues^12,13 may reflect sequestration rather than synthesis of the polypeptide. We show here that several mouse tissues contain preproEGF mRNA and that it is synthesized mainly in the distal tubules of the kidney. PreproEGF does not seem to be processed to EGF or other peptides in this tissue.