The rate of proximal tubular reabsorption of phosphate (P_i) is a major determinant of P_i homeostasis. Deviations of the extracellular concentration of P_iare corrected by many factors that control the activity of Na-P_i cotransport across the apical membrane. In this review, we describe the regulation of proximal tubule P_i reabsorption via one particular Na-P_i cotransporter (the type IIa cotransporter) by parathyroid hormone (PTH) and dietary phosphate intake. Available data indicate that both factors determine the net amount of type IIa protein residing in the apical membrane. The resulting change in transport capacity is a function of both the rate of cotransporter insertion and internalization. The latter process is most likely regulated by PTH and dietary P_i and is considered irreversible since internalized type IIa Na-P_icotransporters are subsequently routed to the lysosomes for degradation.