The effect of hydrocortisone (HC) on the renal Pi transport in the presence and in the absence of parathyroid hormone (PTH) in the rat was examined using clearance techniques. During constant infusion of PTH in parathyroidectomized (PTX) rats, HC decreased the reabsorption of Pi from 2.84 to 2.33 mumol/min (P less than 0.001), the TmPi from 4.55 to 3.83 mumol/min (P less than 0.001), the reabs. Pi/GFR values from 0.93 to 0.74 (P less than 0.001) and the TmPi/GFR from 1.73 to 1.42 mumol/ml (P less than 0.001). In the absence of PTH, HC diminished the TmPi from 8.36 to 6.58 mumol/min (P less than 0.005) and the TmPi/GFR from 3.36 to 2.51 mumol/ml (P less than 0.001). It is concluded that the phosphaturic response of hydrocortisone may be important in the homeostasis of inorganic phosphate in the body.