Renal phosphate reabsorption via the type II sodium/ phosphate cotransporter (NaPi-2) in the brush border membrane (BBM) of proximal tubules underlies alterations during aging. The ontogeny of NaPi-2 in kidneys from newborn to 6-wk-old rats was investigated. NaPi-2 protein distribution in the kidneys of neonatal, 13-d-old, 22-d-old, and 6-wk-old rats was immunohistochemically analyzed, and NaPi-2 mRNA distribution in neonatal and 6-wk-old rats was analyzed by in situ hybridization. In kidneys of newborn rats, the appearance of NaPi-2 protein and mRNA coincided with the development of the brush border (assessed by actin staining) on proximal tubular cells. NaPi-2 was not detectable in the nephrogenic zone or in the outgrowing straight sections of proximal tubules, which lack a brush border. In 13-d-old suckling rats, strong NaPi-2 staining was seen in the BBM of convoluted proximal tubules of all nephron generations. In contrast, in 22-d-old weaned rats, NaPi-2 staining in the BBM of superficial nephrons was weaker than that in the BBM of juxtamedullary nephrons. Western blotting demonstrated that the overall abundance of NaPi-2 protein in the BBM of 22-d-old rats was decreased to approximately 70% of that in 13-d-old rats. In kidneys of 6-wk-old rats, the internephron gradient for NaPi-2 abundance in the BBM corresponded to that in adult rats. The data suggest that the NaPi-2 system in the kidney is fully functional and possesses the capacity for regulation as soon as nephrogenesis is completed. The manifestation of NaPi-2 internephron heterogeneity immediately after weaning might be related to the change in dietary inorganic phosphate content.