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The mRNA expression and localization of the aquaporin (AQP) family in rat kidney were examined by ribonuclease protection assay and immunohistochemistry. AQPl, AQP2, AQP3, and AQP4 mRNA were hardly detectable in 16-day gestation fetuses. AQPl mRNA was explosively expressed at 1 wk, keeping the level throughout life. AQP2 mRNA expression was apparently noticed in l&day fetuses and was enhanced gradually with age to reach a plateau at 4 wk. AQP3 and AQP4 mRNA expression was significantly found at birth but was not changed remarkably thereafter. AQP2 protein appeared first at the apical side of collecting duct cells in 1%day fetuses. The staining intensity at the site increased with age, and basolateral staining was added in adult rats. AQP3 was distinctly demonstrated at the basolateral side of collecting duct cells after birth, and the staining intensity was almost stable throughout life. The progressive induction of AQP2 expression in the first 4 wk after birth is presumed to contribute to the maturation of urinary concentrating capacity during the kidney development. [ABSTRACT FROM AUTHOR]