ABSTRACT.Using a specific and sensitive epidermal growth factor (EGF) radioimmunoassay, we have shown measurable quantities of EGF in mouse urine during the neonatal period. Sephadex G-50 column chromatography demonstrated the presence of a single immunoreactive component at the position defined by standard EGF (mol wt 6045). Comparison of urine urea nitrogen and urine EGF levels in neonatal and adult mice showed adult values to be 3− and 16-fold higher, respectively. Kidney weights relative to body weight were similar in newborn and adult animals while kidney EGF concentration per mg protein was 2.5-fold higher in the adult. The relative submandibular gland (SMG) weight was slightly higher in adult female mice than in the newborn, whereas SMG-EGF concentration was 15,000-fold higher in the adult than in the newborn.Thyroxine administration to neonatal mice from day 0 to day 6 increased urine EGF concentration 7-fold compared to control pups. Though the hormone treatment elicited a significant increase in relative SMG weight, its EGF concentration like that of the kidneys remained unchanged. The results suggest that urine EGF is subject to thyroid hormone modulation in newborn animals and that the changes in urine EGF concentration are independent of changes in SMG and renal EGF levels.