Growth hormone (GH) is an important factor in the developmental adaptation to enhance Pi reabsorption, however, the nephron sites and mechanisms by which GH regulates renal Pi uptake remains unclear and is the focus of the present study. Micropuncture experiments were performed following acute thyroparathyroidectomy in the presence and absence of PTH, in adult (14-17 wks old), juvenile (4 wks old), and GH-suppressed juvenile male rats. While the phosphaturic effect of PTH was blunted in the juvenile rat compared with the adult, suppression of GH in the juvenile restored FEPi to adult levels. In the presence or absence of PTH, GH suppression in the juvenile rat caused a significant increase in the fractional Pi delivery (FDPi) to the late proximal convoluted (PCT) and early distal (eDT) tubule, so that delivery was not different from adults. This data was confirmed by Pi uptake studies into BBM vesicles. Immunofluorescence studies indicate increased BBM NaPi-2 expression in the juvenile compared to adult rat, and GH suppression reduced the NaPi-2 expression to levels observed in the adult. GH replacement in the GRF-AN-treated juveniles restored the high NaPi-2 expression and Pi uptake. Together, these novel results demonstrate that the presence of GH in the juvenile animal is crucial for the early developmental upregulation of BBM type IIa NaPi cotransporters, and most importantly, describe the enhanced Pi reabsorption along the PCT and proximal straight nephron segments in the juvenile rat.