Dietary phosphate depletion is the most powerful known stimulator of renal tubular phosphate transport, the signaling of which has, however, remained incompletely understood. Renal phosphate reabsorption is moderately impaired in gene-targeted mice lacking SGK3 or Akt2. Targets of both kinases include glycogen synthase kinase GSK3, which has previously been shown to inhibit the renal tubular phosphate transporter NaPiIIa. The present study explored, whether PKB/SGK-dependent GSK3 activity contributes to the upregulation of renal tubular phosphate during phosphate depletion. To this end, experiments were performed in mice expressing PKB/SGK-dependent GSK3a,ß (gsk3^KI) and in wild type (gsk3^WT) mice. Prior to phosphate depletion the urinary phosphate excretion was significantly higher in gsk3^KI than in gsk3^WT mice. In gsk3^WT mice the phosphorylation of renal GSK3 was increased by phosphate depletion, an effect paralleled by marked decrease of the urinary phosphate excretion. In gsk3^KI mice, however, the phosphate depletion was followed by an only subtle decrease of phosphate excretion and, hence, by dramatic urinary phosphate loss. Under a normal diet, the plasma 1,25(OH)₂D₃ concentration was lower in gsk3^KI mice than in gsk3^WT mice, a difference becoming even more prominent under the low-phosphate diet. Following phosphate depletion, the plasma concentration of FGF23 was significantly higher in gsk3^KI mice than in gsk3^WT mice. The observations reveal that PKB/SGK-dependent GSK phosphorylation is a critical step in the upregulation of renal tubular phosphate reabsorption and maintenance of phophate balance during dietary phosphate depletion.