Epidermal growth factor (EGF) is important in mammalian renal development. In our study we investigated the detailed distribution and the time of the first appearance of EGF in developing rat kidney. Kidneys from 18-(E18) and 20-day-old (E20) fetuses, 1- (P1), 3- (P3), 7- (P7), 14- (P14), and 21-day-old (P21) pups and adults were processed for immunohistochemistry and electronmicroscopy. In adult rat kidney, EGF immunoreactivity was found in distal tubule including thick ascending limb (TAL) and portion 1 of distal convoluted tubule (DCT1), whereas no EGF immunoreactivity was seen in the portion 2 of distal convoluted tubule (DCT2) and connecting tubule. In developing kidney, EGF-positive cells first appeared at P3 and were localized in the middle portion of the differentiating TAL of the corticomedullary junction. By P7 the abundance of EGF expression had dramatically increased in the medullary TAL. Between 14 and 21 days postnatal, EGF immunoreactivity was found in the TAL and the DCT for the first time. However, EGF-positive and EGF-negative cells were in the TAL in developing rat kidney. EGF-positive cells did not differ from negative cells in the expression of sodium transport proteins or in the proliferation rate at P3 and P7. In the TAL, smooth-surfaced cells had strong EGF-immunoreactivity, but no EGF-immunoreactivity was seen in the rough-surfaced cells with well-developed microvilli. Our results suggest that the expression of EGF in developing kidney plays important role in the regulation of growth and differentiation of the loop of Henle during kidney development, and that may act as paracrine mode.