In the kidney, about 80% of the filtered phosphate (Pi) is reabsorbed along the proximal tubule. Changes in renal Pi reabsorption are associated with modulation of the sodium dependent Pi cotransporter NaPi Type IIa (NaPi-IIa) and Type IIc (NaPi-IIc) protein abundance in the brush border membrane (BBM) of proximal tubule cells. NaPi-IIa is mainly regulated by dietary Pi intake and parathyroid hormone (PTH). The purpose of the present study was to elucidate the effect of alteration in dietary magnesium (Mg(2+)) intake on renal Pi handling. Urinary Pi excretion and renal expression of NaPi-IIa and NaPi-IIc were analyzed in rats fed with normal (0.2%) or high Mg(2+) (2.5%) diet. High Mg(2+) diet resulted in decreased renal Pi excretion and increased protein expression of NaPi-IIa and NaPi-IIc. Serum FGF-23 (fibroblast growth factor 23) levels were unchanged under high Mg(2+) diet. Serum PTH levels were slightly decreased under high Mg(2+) diet. To examine whether the observed changes in renal Pi reabsorption are PTH dependent, NaPi-IIa and NaPi-IIc expression were also analyzed in parathyroidectomized (PTX) rats fed with normal or high Mg(2+) diet. In PTX rats Mg(2+) had no significant effect on renal Pi excretion or NaPi-IIa protein expression. Mg(2+) increased NaPi-IIc protein expression in PTX rats. This experiment shows for the first time on the molecular level how Mg(2+) stimulates renal Pi reabsorption. Under high Mg(2+) diet NaPi-IIa expression is dependent on PTH levels, whereas NaPi-IIc expression seems to be independent from PTH levels.