Intact and parathyroidectomized male Wistar rats weighing 150 to 170 g were pair-fed on diets which contained inorganic P 0.2, 1.2 and 1.8 g/100 g for periods up to 10 days. Clearances were estimated before and after infusions of inorganic P by vein at different concentrations. The renal tubule could adapt its capacity to transport inorganic P according to P in the diet. Tubular reabsorption of phosphate, for a given plasma concentration of inorganic P, was greater in rats on diets low in P than in those on diets high in P. This modification also occurred in the absence of thyroid and parathyroid glands, in the presence of the same blood Ca and urine pH and during extracellular volume expansion. A difference in the tubular handling of inorganic P was detectable as early as 3 days after switching from a low- to a high-P diet. The results show that factors other than parathyrin are implicated in the tubular response to difference in intake of inorganic phosphate.