Renal proximal tubules play a vital role in phosphorus (P) homeostasis. It is well known that dietary P restriction up-regulates the activities of 25-hydroxyvitamin D(3)-1alpha-hydroxylase (1-OHase), an enzyme that is involved in activation of vitamin D and thereby maintaining P balance. However, the mechanism involved in such regulation is not known. In the present study, we aim to identify proteins that might be involved in the renal adaptation to dietary P restriction using a proteomic approach. Renal proximal tubules were harvested from young rats fed either normal P diet or low P diet (LPD) for 1 to 7 days. Western blotting analysis of 1-OHase and signaling proteins in insulin-like growth factor I axis indicated an increase in expression of these proteins upon dietary P restriction. Using two-dimensional electrophoresis, we found that LPD reduced the total number of protein species expressed in renal proximal tubules. Differentially expressed proteins were analyzed and located using the software Melanie III, and their identities were found using matrix-assisted laser desorption/ionization-time of flight mass spectrometry. Our results showed that beta-actin, gamma-actin, major urinary protein, phosphatidylinositol transfer protein beta isoform, and G1/S-specific cyclin D3 are up-regulated and nonspecific lipid transfer protein is down-regulated by LPD.