Recent studies in the developing rat kidney have demonstrated that renin in juxtaglomerular cells and aquaporin-1(AQP1), a kind of water channel, participate in the development of renal arterial system. The purpose of this study was to identify the association among renin and AQP1 in the developing renal arterial system. Sprague-Dawley rat kidneys from 14-, 16-, 18- and 20-day-old fetuses, and 1-, 4-, 7-, 14-, 21- and 28-day-old pups were preserved with periodate-lysine-2% paraformaldehyde solution for single or multiple immunohistochemistry. Renin- positive, smooth muscle, and endothelial cells were detected using renin polyclonal(1 : 5,000), alpha-smooth muscle actin(ASMA) monoclonal(1 : 1,000), and AQP1 polyclonal(1 : 500) antibodies, respectively. Immunoreactivity for renin and AQP1 was not detected in the developing vessels in fetal kidneys on the 14 th day of gestation. At the 16th day of gestation, AQP1 appeared in the developing kidney. Immunoreactivity for AQP1 was observed in endothelial cells of the arterial capillary plexus and of the arcuate artery, but not in the venous capillary plexus or the arcuate vein. At the 18th day of gestation, renin-positive cells appeared throughout the arterial system including the arcuate artery. After birth, immunoreactivity for renin and AQP1 was gradually decreased in the developing artery. No renin immunoreactivity was detected in the arcuate artery from 7 days after birth, and in the interlobular artery from 14 days after birth. Renin immunoreactivity was confined to juxtaglomerular cells in the afferent arteriole from 21 days after birth. AQP1 immunoreactivity in endothelial cells was not observed in the arcuate artery from right after birth, or in the interlobular artery, or afferent arteriole from 14 days after birth. This study indicates that the expression and loss of AQP1 in endothelial cells spatiotemporally coexists with renin in smooth muscle cells during the development of arterial system in the rat kidney. It is suggested that AQP1 and renin might play an important role in the development and growth of the rat kidney arterial system by functioning through a paracrine or autocrine mechanism.