1. The existence of tubular secretion of inorganic phosphate (Pi) in the mammalian kidney has been investigated by studying the renal response of rats infused with sodium phosphate by three different techniques.

2. Clearance studies indicate that, in anaesthetized rats, the net tubular reabsorption decreases markedly in response to Pi infusion. In conscious rats, the clearance of Pi slightly exceeded that of inulin at high plasma Pi concentration.

3. Free-flow micropuncture in control rats showed a net tubular reabsorption of Pi along the proximal tubule, and probably between the end of the distal tubule and the ureteral urine. In phosphate-loaded rats, whether receiving parathyroid hormone or not, an apparent net secretion of Pi was observed between the end of the distal tubule and the ureteral urine. In the phosphate-loaded group receiving parathyroid hormone, net secretion was also observed very early in the proximal tubule followed by a predominant reabsorption along this segment. Thus the early proximal tubule and probably also the terminal nephron can be the site of either net reabsorption or net secretion.

4. Microperfusions of proximal tubules show a fall in the specific radioactivity of the perfused radioactive Pi solution, indicating entry of Pi into the lumen.

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